ref: use user agent roundtripper from prom common

Signed-off-by: TJ Hoplock <[email protected]>

Author: tjhop

pkg/prometheus/api.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/prometheus/client_golang/api"
 	promv1 "github.com/prometheus/client_golang/api/prometheus/v1"
+	"github.com/prometheus/common/config"
 
 	"github.com/tjhop/prometheus-mcp-server/internal/version"
 )
@@ -14,38 +15,12 @@ var (
 	userAgent = fmt.Sprintf("prometheus-mcp-server/%s (https://github.com/tjhop/prometheus-mcp-server)", version.Version)
 )
 
-type userAgentRoundTripper struct {
-	name string
-	rt   http.RoundTripper
-}
-
-// RoundTrip implements the http.RoundTripper interface.
-func (u userAgentRoundTripper) RoundTrip(r *http.Request) (*http.Response, error) {
-	if r.UserAgent() == "" {
-		// The specification of http.RoundTripper says that it shouldn't mutate
-		// the request so make a copy of req.Header since this is all that is
-		// modified.
-		r2 := new(http.Request)
-		*r2 = *r
-		r2.Header = make(http.Header)
-		for k, s := range r.Header {
-			r2.Header[k] = s
-		}
-		r2.Header.Set("User-Agent", u.name)
-		r = r2
-	}
-	return u.rt.RoundTrip(r)
-}
-
 func NewAPIClient(prometheusUrl string, rt http.RoundTripper) (promv1.API, error) {
 	if rt == nil {
 		rt = http.DefaultTransport
 	}
 
-	uart := userAgentRoundTripper{
-		name: userAgent,
-		rt:   rt,
-	}
+	uart := config.NewUserAgentRoundTripper(userAgent, rt)
 
 	client, err := api.NewClient(api.Config{
 		Address:      prometheusUrl,