Add Prometheus Metrics

at the first iteration, only the following metrics are collected:

  - HTTP metrics of each API endpoint
  - cache counter for request/hit/miss
  - histogram of storage actions, including:
    GetContent, PutContent, Stat, List, Move, and Delete

Signed-off-by: tifayuki <tifayuki@gmail.com>

vendor/github.com/prometheus/client_golang/prometheus/promhttp/http.go

@@ -0,0 +1,267 @@
+// Copyright 2016 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package promhttp provides tooling around HTTP servers and clients.
+//
+// First, the package allows the creation of http.Handler instances to expose
+// Prometheus metrics via HTTP. promhttp.Handler acts on the
+// prometheus.DefaultGatherer. With HandlerFor, you can create a handler for a
+// custom registry or anything that implements the Gatherer interface. It also
+// allows the creation of handlers that act differently on errors or allow to
+// log errors.
+//
+// Second, the package provides tooling to instrument instances of http.Handler
+// via middleware. Middleware wrappers follow the naming scheme
+// InstrumentHandlerX, where X describes the intended use of the middleware.
+// See each function's doc comment for specific details.
+//
+// Finally, the package allows for an http.RoundTripper to be instrumented via
+// middleware. Middleware wrappers follow the naming scheme
+// InstrumentRoundTripperX, where X describes the intended use of the
+// middleware. See each function's doc comment for specific details.
+package promhttp
+
+import (
+	"bytes"
+	"compress/gzip"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"sync"
+
+	"github.com/prometheus/common/expfmt"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	contentTypeHeader     = "Content-Type"
+	contentLengthHeader   = "Content-Length"
+	contentEncodingHeader = "Content-Encoding"
+	acceptEncodingHeader  = "Accept-Encoding"
+)
+
+var bufPool sync.Pool
+
+func getBuf() *bytes.Buffer {
+	buf := bufPool.Get()
+	if buf == nil {
+		return &bytes.Buffer{}
+	}
+	return buf.(*bytes.Buffer)
+}
+
+func giveBuf(buf *bytes.Buffer) {
+	buf.Reset()
+	bufPool.Put(buf)
+}
+
+// Handler returns an http.Handler for the prometheus.DefaultGatherer, using
+// default HandlerOpts, i.e. it reports the first error as an HTTP error, it has
+// no error logging, and it applies compression if requested by the client.
+//
+// The returned http.Handler is already instrumented using the
+// InstrumentMetricHandler function and the prometheus.DefaultRegisterer. If you
+// create multiple http.Handlers by separate calls of the Handler function, the
+// metrics used for instrumentation will be shared between them, providing
+// global scrape counts.
+//
+// This function is meant to cover the bulk of basic use cases. If you are doing
+// anything that requires more customization (including using a non-default
+// Gatherer, different instrumentation, and non-default HandlerOpts), use the
+// HandlerFor function. See there for details.
+func Handler() http.Handler {
+	return InstrumentMetricHandler(
+		prometheus.DefaultRegisterer, HandlerFor(prometheus.DefaultGatherer, HandlerOpts{}),
+	)
+}
+
+// HandlerFor returns an uninstrumented http.Handler for the provided
+// Gatherer. The behavior of the Handler is defined by the provided
+// HandlerOpts. Thus, HandlerFor is useful to create http.Handlers for custom
+// Gatherers, with non-default HandlerOpts, and/or with custom (or no)
+// instrumentation. Use the InstrumentMetricHandler function to apply the same
+// kind of instrumentation as it is used by the Handler function.
+func HandlerFor(reg prometheus.Gatherer, opts HandlerOpts) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		mfs, err := reg.Gather()
+		if err != nil {
+			if opts.ErrorLog != nil {
+				opts.ErrorLog.Println("error gathering metrics:", err)
+			}
+			switch opts.ErrorHandling {
+			case PanicOnError:
+				panic(err)
+			case ContinueOnError:
+				if len(mfs) == 0 {
+					http.Error(w, "No metrics gathered, last error:\n\n"+err.Error(), http.StatusInternalServerError)
+					return
+				}
+			case HTTPErrorOnError:
+				http.Error(w, "An error has occurred during metrics gathering:\n\n"+err.Error(), http.StatusInternalServerError)
+				return
+			}
+		}
+
+		contentType := expfmt.Negotiate(req.Header)
+		buf := getBuf()
+		defer giveBuf(buf)
+		writer, encoding := decorateWriter(req, buf, opts.DisableCompression)
+		enc := expfmt.NewEncoder(writer, contentType)
+		var lastErr error
+		for _, mf := range mfs {
+			if err := enc.Encode(mf); err != nil {
+				lastErr = err
+				if opts.ErrorLog != nil {
+					opts.ErrorLog.Println("error encoding metric family:", err)
+				}
+				switch opts.ErrorHandling {
+				case PanicOnError:
+					panic(err)
+				case ContinueOnError:
+					// Handled later.
+				case HTTPErrorOnError:
+					http.Error(w, "An error has occurred during metrics encoding:\n\n"+err.Error(), http.StatusInternalServerError)
+					return
+				}
+			}
+		}
+		if closer, ok := writer.(io.Closer); ok {
+			closer.Close()
+		}
+		if lastErr != nil && buf.Len() == 0 {
+			http.Error(w, "No metrics encoded, last error:\n\n"+lastErr.Error(), http.StatusInternalServerError)
+			return
+		}
+		header := w.Header()
+		header.Set(contentTypeHeader, string(contentType))
+		header.Set(contentLengthHeader, fmt.Sprint(buf.Len()))
+		if encoding != "" {
+			header.Set(contentEncodingHeader, encoding)
+		}
+		w.Write(buf.Bytes())
+		// TODO(beorn7): Consider streaming serving of metrics.
+	})
+}
+
+// InstrumentMetricHandler is usually used with an http.Handler returned by the
+// HandlerFor function. It instruments the provided http.Handler with two
+// metrics: A counter vector "promhttp_metric_handler_requests_total" to count
+// scrapes partitioned by HTTP status code, and a gauge
+// "promhttp_metric_handler_requests_in_flight" to track the number of
+// simultaneous scrapes. This function idempotently registers collectors for
+// both metrics with the provided Registerer. It panics if the registration
+// fails. The provided metrics are useful to see how many scrapes hit the
+// monitored target (which could be from different Prometheus servers or other
+// scrapers), and how often they overlap (which would result in more than one
+// scrape in flight at the same time). Note that the scrapes-in-flight gauge
+// will contain the scrape by which it is exposed, while the scrape counter will
+// only get incremented after the scrape is complete (as only then the status
+// code is known). For tracking scrape durations, use the
+// "scrape_duration_seconds" gauge created by the Prometheus server upon each
+// scrape.
+func InstrumentMetricHandler(reg prometheus.Registerer, handler http.Handler) http.Handler {
+	cnt := prometheus.NewCounterVec(
+		prometheus.CounterOpts{
+			Name: "promhttp_metric_handler_requests_total",
+			Help: "Total number of scrapes by HTTP status code.",
+		},
+		[]string{"code"},
+	)
+	// Initialize the most likely HTTP status codes.
+	cnt.WithLabelValues("200")
+	cnt.WithLabelValues("500")
+	if err := reg.Register(cnt); err != nil {
+		if are, ok := err.(prometheus.AlreadyRegisteredError); ok {
+			cnt = are.ExistingCollector.(*prometheus.CounterVec)
+		} else {
+			panic(err)
+		}
+	}
+
+	gge := prometheus.NewGauge(prometheus.GaugeOpts{
+		Name: "promhttp_metric_handler_requests_in_flight",
+		Help: "Current number of scrapes being served.",
+	})
+	if err := reg.Register(gge); err != nil {
+		if are, ok := err.(prometheus.AlreadyRegisteredError); ok {
+			gge = are.ExistingCollector.(prometheus.Gauge)
+		} else {
+			panic(err)
+		}
+	}
+
+	return InstrumentHandlerCounter(cnt, InstrumentHandlerInFlight(gge, handler))
+}
+
+// HandlerErrorHandling defines how a Handler serving metrics will handle
+// errors.
+type HandlerErrorHandling int
+
+// These constants cause handlers serving metrics to behave as described if
+// errors are encountered.
+const (
+	// Serve an HTTP status code 500 upon the first error
+	// encountered. Report the error message in the body.
+	HTTPErrorOnError HandlerErrorHandling = iota
+	// Ignore errors and try to serve as many metrics as possible.  However,
+	// if no metrics can be served, serve an HTTP status code 500 and the
+	// last error message in the body. Only use this in deliberate "best
+	// effort" metrics collection scenarios. It is recommended to at least
+	// log errors (by providing an ErrorLog in HandlerOpts) to not mask
+	// errors completely.
+	ContinueOnError
+	// Panic upon the first error encountered (useful for "crash only" apps).
+	PanicOnError
+)
+
+// Logger is the minimal interface HandlerOpts needs for logging. Note that
+// log.Logger from the standard library implements this interface, and it is
+// easy to implement by custom loggers, if they don't do so already anyway.
+type Logger interface {
+	Println(v ...interface{})
+}
+
+// HandlerOpts specifies options how to serve metrics via an http.Handler. The
+// zero value of HandlerOpts is a reasonable default.
+type HandlerOpts struct {
+	// ErrorLog specifies an optional logger for errors collecting and
+	// serving metrics. If nil, errors are not logged at all.
+	ErrorLog Logger
+	// ErrorHandling defines how errors are handled. Note that errors are
+	// logged regardless of the configured ErrorHandling provided ErrorLog
+	// is not nil.
+	ErrorHandling HandlerErrorHandling
+	// If DisableCompression is true, the handler will never compress the
+	// response, even if requested by the client.
+	DisableCompression bool
+}
+
+// decorateWriter wraps a writer to handle gzip compression if requested.  It
+// returns the decorated writer and the appropriate "Content-Encoding" header
+// (which is empty if no compression is enabled).
+func decorateWriter(request *http.Request, writer io.Writer, compressionDisabled bool) (io.Writer, string) {
+	if compressionDisabled {
+		return writer, ""
+	}
+	header := request.Header.Get(acceptEncodingHeader)
+	parts := strings.Split(header, ",")
+	for _, part := range parts {
+		part := strings.TrimSpace(part)
+		if part == "gzip" || strings.HasPrefix(part, "gzip;") {
+			return gzip.NewWriter(writer), "gzip"
+		}
+	}
+	return writer, ""
+}