Add support for extended promql functions in rule

Adds a flag to register the extended promql functions supported by the thanos
query engine when running the rule component.  This will allow rule config
files containing query expressions with (xrate / xincrease / xdelta) to pass
validation.  This will only work if the query endpoint in use is running the
thanos engine.

cmd/thanos/rule.go

@@ -34,6 +34,7 @@ import (
 	"github.com/prometheus/prometheus/model/relabel"
 	"github.com/prometheus/prometheus/notifier"
 	"github.com/prometheus/prometheus/promql"
+	"github.com/prometheus/prometheus/promql/parser"
 	"github.com/prometheus/prometheus/rules"
 	"github.com/prometheus/prometheus/storage"
 	"github.com/prometheus/prometheus/storage/remote"
@@ -44,6 +45,7 @@ import (
 	"github.com/thanos-io/objstore"
 	"github.com/thanos-io/objstore/client"
 	objstoretracing "github.com/thanos-io/objstore/tracing/opentracing"
+	"github.com/thanos-io/promql-engine/execution/parse"
 	"gopkg.in/yaml.v2"
 
 	"github.com/thanos-io/thanos/pkg/alert"
@@ -105,6 +107,8 @@ type ruleConfig struct {
 	lset              labels.Labels
 	ignoredLabelNames []string
 	storeRateLimits   store.SeriesSelectLimits
+
+	extendedFunctionsEnabled bool
 }
 
 type Expression struct {
@@ -155,6 +159,8 @@ func registerRule(app *extkingpin.App) {
 	cmd.Flag("grpc-query-endpoint", "Addresses of Thanos gRPC query API servers (repeatable). The scheme may be prefixed with 'dns+' or 'dnssrv+' to detect Thanos API servers through respective DNS lookups.").
 		PlaceHolder("<endpoint>").StringsVar(&conf.grpcQueryEndpoints)
 
+	cmd.Flag("query.enable-x-functions", "Whether to enable extended rate functions (xrate, xincrease and xdelta). Only has effect when used with Thanos engine.").Default("false").BoolVar(&conf.extendedFunctionsEnabled)
+
 	conf.rwConfig = extflag.RegisterPathOrContent(cmd, "remote-write.config", "YAML config for the remote-write configurations, that specify servers where samples should be sent to (see https://prometheus.io/docs/prometheus/latest/configuration/configuration/#remote_write). This automatically enables stateless mode for ruler and no series will be stored in the ruler's TSDB. If an empty config (or file) is provided, the flag is ignored and ruler is run with its own TSDB.", extflag.WithEnvSubstitution())
 
 	conf.objStoreConfig = extkingpin.RegisterCommonObjStoreFlags(cmd, "", false)
@@ -582,6 +588,12 @@ func runRule(
 		alertQ  = alert.NewQueue(logger, reg, 10000, 100, labelsTSDBToProm(conf.lset), conf.alertmgr.alertExcludeLabels, alertRelabelConfigs)
 	)
 	{
+		if conf.extendedFunctionsEnabled {
+			for k, fn := range parse.XFunctions {
+				parser.Functions[k] = fn
+			}
+		}
+
 		// Run rule evaluation and alert notifications.
 		notifyFunc := func(ctx context.Context, expr string, alerts ...*rules.Alert) {
 			res := make([]*notifier.Alert, 0, len(alerts))