[#493] Refactor serving of prometheus and pprof services

Rename `util/profiler` package to `httputil` and refactor it:

  * simplify utility HTTP server;

  * make more generic server's parameters in order to remove `viper.Viper`
    dependency;

  * use single constructor for creating the pprof and prometheus servers;

  * replace `enabled` config value with empty-check of the network address.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/util/http/server.go

@@ -0,0 +1,87 @@
+package httputil
+
+import (
+	"fmt"
+	"net/http"
+	"time"
+)
+
+// Prm groups the required parameters of the Server's constructor.
+//
+// All values must comply with the requirements imposed on them.
+// Passing incorrect parameter values will result in constructor
+// failure (error or panic depending on the implementation).
+type Prm struct {
+	// TCP address for the server to listen on.
+	//
+	// Must be a valid TCP address.
+	Address string
+
+	// Must not be nil.
+	Handler http.Handler
+}
+
+// Server represents a wrapper over http.Server
+// that provides interface to start and stop
+// listening routine.
+//
+// For correct operation, Server must be created
+// using the constructor (New) based on the required parameters
+// and optional components. After successful creation,
+// Server is immediately ready to work through API.
+type Server struct {
+	shutdownTimeout time.Duration
+
+	srv *http.Server
+}
+
+const invalidValFmt = "invalid %s %s (%T): %v"
+
+func panicOnPrmValue(n string, v interface{}) {
+	panicOnValue("parameter", n, v)
+}
+
+func panicOnOptValue(n string, v interface{}) {
+	panicOnValue("option", n, v)
+}
+
+func panicOnValue(t, n string, v interface{}) {
+	panic(fmt.Sprintf(invalidValFmt, t, n, v, v))
+}
+
+// New creates a new instance of the Server.
+//
+// Panics if at least one value of the parameters is invalid.
+//
+// Panics if at least one of next optinal parameters is invalid:
+//  * shutdown timeout is non-positive.
+//
+// The created Server does not require additional
+// initialization and is completely ready for work.
+func New(prm Prm, opts ...Option) *Server {
+	switch {
+	case prm.Address == "":
+		panicOnPrmValue("Address", prm.Address)
+	case prm.Handler == nil:
+		panicOnPrmValue("Handler", prm.Handler)
+	}
+
+	c := defaultCfg()
+
+	for _, o := range opts {
+		o(c)
+	}
+
+	switch {
+	case c.shutdownTimeout <= 0:
+		panicOnOptValue("shutdown timeout", c.shutdownTimeout)
+	}
+
+	return &Server{
+		shutdownTimeout: c.shutdownTimeout,
+		srv: &http.Server{
+			Addr:    prm.Address,
+			Handler: prm.Handler,
+		},
+	}
+}