fix(distributor): relax content-type check

We used to allow content type == text/plain for Prometheus
remote write v1 push. Let's continue to not impact users.

Signed-off-by: György Krajcsovits <[email protected]>

Author: krajorama

CHANGELOG.md

@@ -95,6 +95,7 @@
 * [BUGFIX] Querier: fix issue where `label_join` could incorrectly return multiple series with the same labels rather than failing with `vector cannot contain metrics with the same labelset`. https://github.com/prometheus/prometheus/pull/15975 #10826
 * [BUGFIX] Querier: fix issue where counter resets on native histograms could be incorrectly under- or over-counted when using subqueries. https://github.com/prometheus/prometheus/pull/15987 #10871
 * [BUGFIX] OTLP: Fix response body and Content-Type header to align with spec. #10852
+* [BUGFIX] Distributor: do not reject Prometheus remote write 1.0 with wrong content-type. #10916
 
 ### Mixin
 

pkg/distributor/push.go

@@ -251,7 +251,9 @@ func isRemoteWrite2(r *http.Request) (bool, error) {
 	}
 	parts := strings.Split(contentType, ";")
 	if parts[0] != appProtoContentType {
-		return false, fmt.Errorf("expected %v as the first (media) part, got %v content-type", appProtoContentType, contentType)
+		// We didn't use to check the content type so if someone wrote for
+		// example text/plain here, we'll accept and assume it is v1.
+		return false, nil
 	}
 
 	// Parse potential https://www.rfc-editor.org/rfc/rfc9110#parameter