[#13] Rename go module name according to NSPCC standards

- refactoring s3 gate structure
- cleanup unused code
- rename go module to `github.com/nspcc-dev/neofs-s3-gate`

closes #13

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

api/metrics/collector.go

@@ -0,0 +1,70 @@
+package metrics
+
+import (
+	"github.com/nspcc-dev/neofs-s3-gate/misc"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type stats struct {
+	desc *prometheus.Desc
+}
+
+var (
+	versionInfo = prometheus.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Namespace: "neofs_s3",
+			Name:      "version_info",
+			Help:      "Version of current NeoFS S3 Gate instance",
+		},
+		[]string{
+			// current version
+			"version",
+			// build time of the current version
+			"build_time",
+		},
+	)
+
+	statsMetrics = &stats{
+		desc: prometheus.NewDesc("neofs_s3_stats", "Statistics exposed by MinIO server", nil, nil),
+	}
+)
+
+func init() {
+	prometheus.MustRegister(versionInfo)
+	prometheus.MustRegister(statsMetrics)
+	prometheus.MustRegister(httpRequestsDuration)
+}
+
+func collectNetworkMetrics(ch chan<- prometheus.Metric) {
+	// Network Sent/Received Bytes (Outbound)
+	ch <- prometheus.MustNewConstMetric(
+		prometheus.NewDesc(
+			prometheus.BuildFQName("neofs_s3", "tx", "bytes_total"),
+			"Total number of bytes sent by current NeoFS S3 Gate instance",
+			nil, nil),
+		prometheus.CounterValue,
+		float64(httpStatsMetric.getInputBytes()),
+	)
+
+	ch <- prometheus.MustNewConstMetric(
+		prometheus.NewDesc(
+			prometheus.BuildFQName("neofs_s3", "rx", "bytes_total"),
+			"Total number of bytes received by current NeoFS S3 Gate instance",
+			nil, nil),
+		prometheus.CounterValue,
+		float64(httpStatsMetric.getOutputBytes()),
+	)
+}
+
+func (s *stats) Describe(ch chan<- *prometheus.Desc) {
+	ch <- s.desc
+}
+
+func (s *stats) Collect(ch chan<- prometheus.Metric) {
+	// Expose current version information
+	versionInfo.WithLabelValues(misc.Version, misc.Build).Set(1.0)
+
+	// connect collectors
+	collectHTTPMetrics(ch)
+	collectNetworkMetrics(ch)
+}