Add reading bearer token from a file to http input (influxdata#7304)

plugins/inputs/http/README.md

@@ -26,6 +26,10 @@ The HTTP input plugin collects metrics from one or more HTTP(S) endpoints.  The
   ## compress body or "identity" to apply no encoding.
   # content_encoding = "identity"
 
+  ## Optional file with Bearer token
+  ## file content is added as an Authorization header
+  # bearer_token = "/path/to/file"
+
   ## Optional HTTP Basic Auth Credentials
   # username = "username"
   # password = "pa$$word"

plugins/inputs/http/http.go

@@ -29,6 +29,9 @@ type HTTP struct {
 	Password string `toml:"password"`
 	tls.ClientConfig
 
+	// Absolute path to file with Bearer token
+	BearerToken string `toml:"bearer_token"`
+
 	SuccessStatusCodes []int `toml:"success_status_codes"`
 
 	Timeout internal.Duration `toml:"timeout"`
@@ -52,6 +55,10 @@ var sampleConfig = `
   ## Optional HTTP headers
   # headers = {"X-Special-Header" = "Special-Value"}
 
+  ## Optional file with Bearer token
+  ## file content is added as an Authorization header
+  # bearer_token = "/path/to/file"
+
   ## Optional HTTP Basic Auth Credentials
   # username = "username"
   # password = "pa$$word"
@@ -160,6 +167,15 @@ func (h *HTTP) gatherURL(
 		return err
 	}
 
+	if h.BearerToken != "" {
+		token, err := ioutil.ReadFile(h.BearerToken)
+		if err != nil {
+			return err
+		}
+		bearer := "Bearer " + strings.Trim(string(token), "\n")
+		request.Header.Set("Authorization", bearer)
+	}
+
 	if h.ContentEncoding == "gzip" {
 		request.Header.Set("Content-Encoding", "gzip")
 	}