Make Query requests idempotent

Address prometheus#1020.

Signed-off-by: Tomáš Dohnálek <dohnto@gmail.com>

api/prometheus/v1/api.go

@@ -1138,6 +1138,9 @@ func (h *apiClientImpl) DoGetFallback(ctx context.Context, u *url.URL, args url.
 		return nil, nil, nil, err
 	}
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	// Underlying `net.http` library automatically retries` idempotent requests when connectivity issues are hit.
+	// POST requests are not considered idempotent by default, so we need to explicitly mark them as such.
+	req.Header.Set("Idempotency-Key", "TODO")
 
 	resp, body, warnings, err := h.Do(ctx, req)
 	if resp != nil && (resp.StatusCode == http.StatusMethodNotAllowed || resp.StatusCode == http.StatusNotImplemented) {