make changes as per the doc

pkg/plugin/implementation/otelmetrics/cmd/plugin.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/beckn-one/beckn-onix/pkg/plugin/implementation/otelmetrics"
+)
+
+type middlewareProvider struct{}
+
+func (middlewareProvider) New(ctx context.Context, cfg map[string]string) (func(http.Handler) http.Handler, error) {
+	mw, err := otelmetrics.New(ctx, cfg)
+	if err != nil {
+		return nil, err
+	}
+	return mw.Handler, nil
+}
+
+// Provider is exported for plugin loader.
+var Provider = middlewareProvider{}