prometheus plugin support read bearer token file

conf/input.prometheus/prometheus.toml

@@ -6,7 +6,10 @@
 #     "http://localhost:9104/metrics"
 # ]
 
-# bearer_token = ""
+# bearer_token_string = ""
+
+# e.g. /run/secrets/kubernetes.io/serviceaccount/token
+# bearer_token_file = ""
 
 # # basic auth
 # username = ""

inputs/prometheus/prometheus.go

@@ -6,6 +6,8 @@ import (
 	"log"
 	"net/http"
 	"net/url"
+	"os"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -26,7 +28,8 @@ type Instance struct {
 	URLs              []string          `toml:"urls"`
 	Labels            map[string]string `toml:"labels"`
 	IntervalTimes     int64             `toml:"interval_times"`
-	BearerToken     string            `toml:"bearer_token"`
+	BearerTokenString string            `toml:"bearer_token_string"`
+	BearerTokeFile    string            `toml:"bearer_token_file"`
 	Username          string            `toml:"username"`
 	Password          string            `toml:"password"`
 	Timeout           config.Duration   `toml:"timeout"`
@@ -216,8 +219,18 @@ func (ins *Instance) setHeaders(req *http.Request) {
 		req.SetBasicAuth(ins.Username, ins.Password)
 	}
 
-	if ins.BearerToken != "" {
-		req.Header.Set("Authorization", "Bearer "+ins.BearerToken)
+	if ins.BearerTokeFile != "" {
+		content, err := os.ReadFile(ins.BearerTokeFile)
+		if err != nil {
+			log.Println("E! failed to read bearer token file:", ins.BearerTokeFile, "error:", err)
+			return
+		}
+
+		ins.BearerTokenString = strings.TrimSpace(string(content))
+	}
+
+	if ins.BearerTokenString != "" {
+		req.Header.Set("Authorization", "Bearer "+ins.BearerTokenString)
 	}
 
 	req.Header.Set("Accept", acceptHeader)