prometheus: add middleware function

Add middleware function to track request count
and measure the latency of compose requests.

internal/prometheus/middleware.go

@@ -0,0 +1,20 @@
+package prometheus
+
+import (
+	"strings"
+
+	"github.com/labstack/echo/v4"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+func MetricsMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
+	return func(ctx echo.Context) error {
+		TotalRequests.Inc()
+		if strings.HasSuffix(ctx.Path(), "/compose") {
+			ComposeRequests.Inc()
+		}
+		timer := prometheus.NewTimer(httpDuration.WithLabelValues(ctx.Path()))
+		defer timer.ObserveDuration()
+		return next(ctx)
+	}
+}