refactor: remove ability to set downstream-url in query-frontend (#12191)

#### What this PR does

Remove the ability to set a downstream-url in the query-frontend which
allowed the query-frontend to act as a frontend for any Prometheus
compatible server. The functionality is not used internally and
complicates maintenance of the query-frontend.

#### Which issue(s) this PR fixes or relates to

Fixes #11883

#### Checklist

- [X] Tests updated.
- [X] Documentation added.
- [x] `CHANGELOG.md` updated - the order of entries should be
`[CHANGE]`, `[FEATURE]`, `[ENHANCEMENT]`, `[BUGFIX]`. If changelog entry
is not needed, please add the `changelog-not-needed` label to the PR.
- [ ]
[`about-versioning.md`](https://github.com/grafana/mimir/blob/main/docs/sources/mimir/configure/about-versioning.md)
updated with experimental features.

Signed-off-by: Nick Pillitteri <nick.pillitteri@grafana.com>

Author: 56quarters

CHANGELOG.md

@@ -8,6 +8,7 @@
 * [CHANGE] Querier: Add support for UTF-8 label and metric names in `label_join`, `label_replace` and `count_values` PromQL functions. #11848.
 * [CHANGE] Remove support for Redis as a cache backend. #12163
 * [CHANGE] Memcached: Remove experimental `-<prefix>.memcached.addresses-provider` flag to use alternate DNS service discovery backends. The more reliable backend introduced in 2.16.0 (#10895) is now the default. As a result of this change, DNS-based cache service discovery no longer supports search domains. #12175
+* [CHANGE] Query-frontend: Remove the CLI flag `-query-frontend.downstream-url` and corresponding YAML configuration and the ability to use the query-frontend to proxy arbitrary Prometheus backends. #12191
 * [FEATURE] Distributor: Add experimental `-distributor.otel-native-delta-ingestion` option to allow primitive delta metrics ingestion via the OTLP endpoint. #11631
 * [FEATURE] MQE: Add support for experimental `sort_by_label` and `sort_by_label_desc` PromQL functions. #11930
 * [FEATURE] Ingester/Block-builder: Handle the created timestamp field for remote-write requests. #11977

cmd/mimir/config-descriptor.json

@@ -7494,17 +7494,6 @@
           "fieldFlag": "query-frontend.cache-samples-processed-stats",
           "fieldType": "boolean"
         },
-        {
-          "kind": "field",
-          "name": "downstream_url",
-          "required": false,
-          "desc": "URL of downstream Prometheus.",
-          "fieldValue": null,
-          "fieldDefaultValue": "",
-          "fieldFlag": "query-frontend.downstream-url",
-          "fieldType": "string",
-          "fieldCategory": "advanced"
-        },
         {
           "kind": "block",
           "name": "client_cluster_validation",

cmd/mimir/help-all.txt.tmpl

@@ -2187,8 +2187,6 @@ Usage of ./cmd/mimir/mimir:
     	Cache requests that are not step-aligned.
   -query-frontend.client-cluster-validation.label string
     	[experimental] Optionally define the cluster validation label.
-  -query-frontend.downstream-url string
-    	URL of downstream Prometheus.
   -query-frontend.enable-query-engine-fallback
     	[experimental] If set to true and the Mimir query engine is in use, fall back to using the Prometheus query engine for any queries not supported by the Mimir query engine. (default true)
   -query-frontend.enabled-promql-experimental-functions comma-separated-list-of-strings

docs/sources/mimir/configure/configuration-parameters/index.md

@@ -1826,10 +1826,6 @@ results_cache:
 # CLI flag: -query-frontend.cache-samples-processed-stats
 [cache_samples_processed_stats: <boolean> | default = false]
 
-# (advanced) URL of downstream Prometheus.
-# CLI flag: -query-frontend.downstream-url
-[downstream_url: <string> | default = ""]
-
 client_cluster_validation:
   # (experimental) Optionally define the cluster validation label.
   # CLI flag: -query-frontend.client-cluster-validation.label

pkg/frontend/config.go

@@ -34,8 +34,6 @@ type CombinedFrontendConfig struct {
 
 	QueryMiddleware querymiddleware.Config `yaml:",inline"`
 
-	DownstreamURL string `yaml:"downstream_url" category:"advanced"`
-
 	ClusterValidationConfig clusterutil.ClusterValidationConfig `yaml:"client_cluster_validation" category:"experimental"`
 
 	QueryEngine               string `yaml:"query_engine" category:"experimental"`
@@ -49,7 +47,6 @@ func (cfg *CombinedFrontendConfig) RegisterFlags(f *flag.FlagSet, logger log.Log
 	cfg.QueryMiddleware.RegisterFlags(f)
 	cfg.ClusterValidationConfig.RegisterFlagsWithPrefix("query-frontend.client-cluster-validation.", f)
 
-	f.StringVar(&cfg.DownstreamURL, "query-frontend.downstream-url", "", "URL of downstream Prometheus.")
 	f.StringVar(&cfg.QueryEngine, "query-frontend.query-engine", querier.PrometheusEngine, fmt.Sprintf("Query engine to use, either '%v' or '%v'", querier.PrometheusEngine, querier.MimirEngine))
 	f.BoolVar(&cfg.EnableQueryEngineFallback, "query-frontend.enable-query-engine-fallback", true, "If set to true and the Mimir query engine is in use, fall back to using the Prometheus query engine for any queries not supported by the Mimir query engine.")
 }
@@ -80,11 +77,6 @@ func InitFrontend(
 	codec querymiddleware.Codec,
 ) (http.RoundTripper, *v1.Frontend, *v2.Frontend, error) {
 	switch {
-	case cfg.DownstreamURL != "":
-		// If the user has specified a downstream Prometheus, then we should use that.
-		rt, err := NewDownstreamRoundTripper(cfg.DownstreamURL)
-		return httpRoundTripper(cfg, rt, reg, log), nil, nil, err
-
 	case cfg.FrontendV2.SchedulerAddress != "" || cfg.FrontendV2.QuerySchedulerDiscovery.Mode == schedulerdiscovery.ModeRing:
 		// Query-scheduler is enabled when its address is configured or ring-based service discovery is configured.
 		if cfg.FrontendV2.Addr == "" {
@@ -118,16 +110,6 @@ func invalidClusterValidationReporter(cfg CombinedFrontendConfig, reg prometheus
 	return util.NewInvalidClusterValidationReporter(cfg.ClusterValidationConfig.Label, invalidClusterValidation, logger)
 }
 
-func httpRoundTripper(cfg CombinedFrontendConfig, rt http.RoundTripper, reg prometheus.Registerer, logger log.Logger) http.RoundTripper {
-	if rt == nil {
-		return nil
-	}
-	if cfg.ClusterValidationConfig.Label != "" {
-		return middleware.ClusterValidationRoundTripper(cfg.ClusterValidationConfig.Label, invalidClusterValidationReporter(cfg, reg, logger), rt)
-	}
-	return rt
-}
-
 func grpcToHTTPRoundTripper(cfg CombinedFrontendConfig, grpcRoundTripper transport.GrpcRoundTripper, reg prometheus.Registerer, logger log.Logger) http.RoundTripper {
 	if grpcRoundTripper == nil {
 		return nil