Implement Query API discovery

A recent commit (thanos-io#5250) added a GRPC API to Thanos Query which allows
executing PromQL over GRPC. This API is currently not discoverable
through endpointsets which makes it hard for other Thanos components
to use it.

This commit extends endpointsets with a GetQueryAPIClients method
which returns Query API clients to all components which support
this API.

cmd/thanos/query.go

@@ -675,6 +675,7 @@ func runQuery(
 			info.WithRulesInfoFunc(),
 			info.WithMetricMetadataInfoFunc(),
 			info.WithTargetsInfoFunc(),
+			info.WithQueryAPIInfoFunc(),
 		)
 
 		grpcAPI := apiv1.NewGRPCAPI(time.Now, queryableCreator, engineCreator, instantDefaultMaxSourceResolution)

pkg/info/info.go

@@ -25,6 +25,7 @@ type InfoServer struct {
 	getRulesInfo          func() *infopb.RulesInfo
 	getTargetsInfo        func() *infopb.TargetsInfo
 	getMetricMetadataInfo func() *infopb.MetricMetadataInfo
+	getQueryAPIInfo       func() *infopb.QueryAPIInfo
 }
 
 // NewInfoServer creates a new server instance for given component
@@ -42,6 +43,7 @@ func NewInfoServer(
 		getRulesInfo:          func() *infopb.RulesInfo { return nil },
 		getTargetsInfo:        func() *infopb.TargetsInfo { return nil },
 		getMetricMetadataInfo: func() *infopb.MetricMetadataInfo { return nil },
+		getQueryAPIInfo:       func() *infopb.QueryAPIInfo { return nil },
 	}
 
 	for _, o := range options {
@@ -144,6 +146,21 @@ func WithMetricMetadataInfoFunc(getMetricMetadataInfo ...func() *infopb.MetricMe
 	}
 }
 
+// WithQueryAPIInfoFunc determines the function that should be executed to obtain
+// the query information. If no function is provided, the default empty
+// query info is returned. Only the first function from the list is considered.
+func WithQueryAPIInfoFunc(queryInfo ...func() *infopb.QueryAPIInfo) ServerOptionFunc {
+	if len(queryInfo) == 0 {
+		return func(s *InfoServer) {
+			s.getQueryAPIInfo = func() *infopb.QueryAPIInfo { return &infopb.QueryAPIInfo{} }
+		}
+	}
+
+	return func(s *InfoServer) {
+		s.getQueryAPIInfo = queryInfo[0]
+	}
+}
+
 // RegisterInfoServer registers the info server.
 func RegisterInfoServer(infoSrv infopb.InfoServer) func(*grpc.Server) {
 	return func(s *grpc.Server) {
@@ -161,5 +178,6 @@ func (srv *InfoServer) Info(ctx context.Context, req *infopb.InfoRequest) (*info
 		Rules:          srv.getRulesInfo(),
 		Targets:        srv.getTargetsInfo(),
 		MetricMetadata: srv.getMetricMetadataInfo(),
+		Query:          srv.getQueryAPIInfo(),
 	}, nil
 }

pkg/info/infopb/rpc.proto

@@ -31,21 +31,24 @@ message InfoRequest {}
 message InfoResponse {
     repeated ZLabelSet label_sets      = 1 [(gogoproto.nullable) = false];
     string ComponentType               = 2;
-    
+
     // StoreInfo holds the metadata related to Store API if exposed by the component otherwise it will be null.
-    StoreInfo store                    = 3;    
+    StoreInfo store                    = 3;
 
     // RulesInfo holds the metadata related to Rules API if exposed by the component otherwise it will be null.
     RulesInfo rules                    = 4;
-    
+
     // MetricMetadataInfo holds the metadata related to Metadata API if exposed by the component otherwise it will be null.
     MetricMetadataInfo metric_metadata = 5;
-    
+
     // TargetsInfo holds the metadata related to Targets API if exposed by the component otherwise it will be null.
     TargetsInfo targets                = 6;
-    
+
     // ExemplarsInfo holds the metadata related to Exemplars API if exposed by the component otherwise it will be null.
     ExemplarsInfo exemplars            = 7;
+
+    // QueryAPIInfo holds the metadata related to Query API if exposed by the component, otherwise it will be null.
+    QueryAPIInfo query                 = 8;
 }
 
 // StoreInfo holds the metadata related to Store API exposed by the component.
@@ -70,4 +73,8 @@ message TargetsInfo {
 message ExemplarsInfo {
     int64 min_time = 1;
     int64 max_time = 2;
-}
+}
+
+// QueryInfo holds the metadata related to Query API exposed by the component.
+message QueryAPIInfo {
+}