[coordinator] Add headers to along with request to remote write forwa…

…rd target (#2249)

src/query/api/v1/handler/prometheus/handleroptions/options.go

@@ -40,4 +40,6 @@ type PromWriteHandlerForwardTargetOptions struct {
 	URL string `yaml:"url"`
 	// Method defaults to POST if not set.
 	Method string `yaml:"method"`
+	// Headers to send along with requests to the target.
+	Headers map[string]string `yaml:"headers"`
 }

src/query/api/v1/handler/prometheus/remote/write.go

@@ -425,6 +425,13 @@ func (h *PromWriteHandler) forward(
 		return err
 	}
 
+	if headers := target.Headers; headers != nil {
+		// If headers set, attach to request.
+		for name, value := range headers {
+			req.Header.Add(name, value)
+		}
+	}
+
 	resp, err := h.forwardHTTPClient.Do(req.WithContext(ctx))
 	if err != nil {
 		return err