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 this 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

@@ -675,9 +675,10 @@ func runQuery(
 			info.WithRulesInfoFunc(),
 			info.WithMetricMetadataInfoFunc(),
 			info.WithTargetsInfoFunc(),
+			info.WithQueryAPIInfoFunc(),
 		)
 
-		grpcAPI := apiv1.NewGRPCAPI(time.Now, queryableCreator, engineCreator, instantDefaultMaxSourceResolution)
+		grpcAPI := apiv1.NewGRPCAPI(logger, time.Now, queryableCreator, engineCreator, instantDefaultMaxSourceResolution)
 		s := grpcserver.New(logger, reg, tracer, grpcLogOpts, tagOpts, comp, grpcProbe,
 			grpcserver.WithServer(apiv1.RegisterQueryServer(grpcAPI)),
 			grpcserver.WithServer(store.RegisterStoreServer(proxy)),

pkg/api/query/grpc.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"time"
 
+	"github.com/go-kit/log"
 	"github.com/prometheus/prometheus/promql"
 	"github.com/thanos-io/thanos/pkg/api/query/querypb"
 	"github.com/thanos-io/thanos/pkg/query"
@@ -16,14 +17,16 @@ import (
 )
 
 type GRPCAPI struct {
+	logger                      log.Logger
 	now                         func() time.Time
 	queryableCreate             query.QueryableCreator
 	queryEngine                 func(int64) *promql.Engine
 	defaultMaxResolutionSeconds time.Duration
 }
 
-func NewGRPCAPI(now func() time.Time, creator query.QueryableCreator, queryEngine func(int64) *promql.Engine, defaultMaxResolutionSeconds time.Duration) *GRPCAPI {
+func NewGRPCAPI(logger log.Logger, now func() time.Time, creator query.QueryableCreator, queryEngine func(int64) *promql.Engine, defaultMaxResolutionSeconds time.Duration) *GRPCAPI {
 	return &GRPCAPI{
+		logger:                      logger,
 		now:                         now,
 		queryableCreate:             creator,
 		queryEngine:                 queryEngine,

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
 }