[proxy] refactor metrics and add usage logs (#5533)

* **New Features**
  * Access logs now include bytes_upload and bytes_download (API and schemas updated, fields required).
  * Certificate issuance duration is now recorded as a metric.

* **Refactor**
  * Metrics switched from Prometheus client to OpenTelemetry-backed meters; health endpoint now exposes OpenMetrics via OTLP exporter.

* **Tests**
  * Metric tests updated to use OpenTelemetry Prometheus exporter and MeterProvider.

proxy/internal/accesslog/statuswriter.go

@@ -1,18 +1,39 @@
 package accesslog
 
 import (
+	"io"
+
 	"github.com/netbirdio/netbird/proxy/internal/responsewriter"
 )
 
-// statusWriter captures the HTTP status code from WriteHeader calls.
+// statusWriter captures the HTTP status code and bytes written from responses.
 // It embeds responsewriter.PassthroughWriter which handles all the optional
 // interfaces (Hijacker, Flusher, Pusher) automatically.
 type statusWriter struct {
 	*responsewriter.PassthroughWriter
-	status int
+	status       int
+	bytesWritten int64
 }
 
 func (w *statusWriter) WriteHeader(status int) {
 	w.status = status
 	w.PassthroughWriter.WriteHeader(status)
 }
+
+func (w *statusWriter) Write(b []byte) (int, error) {
+	n, err := w.PassthroughWriter.Write(b)
+	w.bytesWritten += int64(n)
+	return n, err
+}
+
+// bodyCounter wraps an io.ReadCloser and counts bytes read from the request body.
+type bodyCounter struct {
+	io.ReadCloser
+	bytesRead *int64
+}
+
+func (bc *bodyCounter) Read(p []byte) (int, error) {
+	n, err := bc.ReadCloser.Read(p)
+	*bc.bytesRead += int64(n)
+	return n, err
+}