Set user agent when scraping prom metrics (#9271)

plugins/inputs/prometheus/prometheus.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/config"
+	"github.com/influxdata/telegraf/internal"
 	"github.com/influxdata/telegraf/plugins/common/tls"
 	"github.com/influxdata/telegraf/plugins/inputs"
 	parser_v2 "github.com/influxdata/telegraf/plugins/parsers/prometheus"
@@ -58,7 +59,8 @@ type Prometheus struct {
 
 	Log telegraf.Logger
 
-	client *http.Client
+	client  *http.Client
+	headers map[string]string
 
 	// Should we scrape Kubernetes services for prometheus annotations
 	MonitorPods       bool   `toml:"monitor_kubernetes_pods"`
@@ -273,6 +275,10 @@ func (p *Prometheus) Gather(acc telegraf.Accumulator) error {
 			return err
 		}
 		p.client = client
+		p.headers = map[string]string{
+			"User-Agent": internal.ProductToken(),
+			"Accept":     acceptHeader,
+		}
 	}
 
 	var wg sync.WaitGroup
@@ -350,7 +356,7 @@ func (p *Prometheus) gatherURL(u URLAndAddress, acc telegraf.Accumulator) error
 		}
 	}
 
-	req.Header.Add("Accept", acceptHeader)
+	p.addHeaders(req)
 
 	if p.BearerToken != "" {
 		token, err := ioutil.ReadFile(p.BearerToken)
@@ -427,6 +433,12 @@ func (p *Prometheus) gatherURL(u URLAndAddress, acc telegraf.Accumulator) error
 	return nil
 }
 
+func (p *Prometheus) addHeaders(req *http.Request) {
+	for header, value := range p.headers {
+		req.Header.Add(header, value)
+	}
+}
+
 /* Check if the field selector specified is valid.
  * See ToSelectableFields() for list of fields that are selectable:
  * https://github.com/kubernetes/kubernetes/release-1.20/pkg/registry/core/pod/strategy.go