[receiver/prometheus] syncTargetAllocator now detects regex changes i…

…n relabel config (#32127)

**Description:** <Describe what has changed.>
Fixing a bug - With the Targetallocator component in prometheus
receiver, when prometheus scrape config is updated for metric relabel
config with just regex change, the prometheus metrics receiver doesn't
update the hash and hence doesn't pick up the metrics relabel config
with the new regex.
This is because the **regex** struct has unexported fields. This fix
similar to the fix made in
[opentelemetry-operator](open-telemetry/opentelemetry-operator#2171)
fixes this issue by taking into account the unexported fields as well.


**Link to tracking Issue:** - #29313 

**Testing:** Tested to make sure that just the regex changes in metric
relabeling gets picked up with TargetAllocator enabled.

Reopening this PR since the old
one(#30258)
got closed due to inactivity.

---------

Co-authored-by: David Ashpole <[email protected]>

.chloggen/fix_promTAHashComputation.yaml

@@ -0,0 +1,6 @@
+change_type: 'bug_fix'
+component: 'prometheusreceiver'
+note: Fix hash computation to include non exported fields like regex in scrape configuration for TargetAllocator
+issues: [29313]
+subtext:
+change_logs: []

cmd/otelcontribcol/go.mod

@@ -598,7 +598,6 @@ require (
 	github.com/mitchellh/go-homedir v1.1.0 // indirect
 	github.com/mitchellh/go-ps v1.0.0 // indirect
 	github.com/mitchellh/hashstructure v1.1.0 // indirect
-	github.com/mitchellh/hashstructure/v2 v2.0.2 // indirect
 	github.com/mitchellh/mapstructure v1.5.1-0.20231216201459-8508981c8b6c // indirect
 	github.com/mitchellh/reflectwalk v1.0.2 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect

cmd/oteltestbedcol/go.mod

@@ -169,7 +169,6 @@ require (
 	github.com/miekg/dns v1.1.59 // indirect
 	github.com/mitchellh/copystructure v1.2.0 // indirect
 	github.com/mitchellh/go-homedir v1.1.0 // indirect
-	github.com/mitchellh/hashstructure/v2 v2.0.2 // indirect
 	github.com/mitchellh/mapstructure v1.5.1-0.20231216201459-8508981c8b6c // indirect
 	github.com/mitchellh/reflectwalk v1.0.2 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect

exporter/datadogexporter/go.mod

@@ -244,7 +244,6 @@ require (
 	github.com/miekg/dns v1.1.59 // indirect
 	github.com/mitchellh/copystructure v1.2.0 // indirect
 	github.com/mitchellh/go-homedir v1.1.0 // indirect
-	github.com/mitchellh/hashstructure/v2 v2.0.2 // indirect
 	github.com/mitchellh/mapstructure v1.5.1-0.20231216201459-8508981c8b6c // indirect
 	github.com/mitchellh/reflectwalk v1.0.2 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect

exporter/prometheusexporter/go.mod

@@ -118,7 +118,6 @@ require (
 	github.com/miekg/dns v1.1.59 // indirect
 	github.com/mitchellh/copystructure v1.2.0 // indirect
 	github.com/mitchellh/go-homedir v1.1.0 // indirect
-	github.com/mitchellh/hashstructure/v2 v2.0.2 // indirect
 	github.com/mitchellh/mapstructure v1.5.1-0.20231216201459-8508981c8b6c // indirect
 	github.com/mitchellh/reflectwalk v1.0.2 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect

receiver/prometheusreceiver/go.mod

@@ -6,7 +6,6 @@ require (
 	github.com/go-kit/log v0.2.1
 	github.com/gogo/protobuf v1.3.2
 	github.com/golang/snappy v0.0.4
-	github.com/mitchellh/hashstructure/v2 v2.0.2
 	github.com/open-telemetry/opentelemetry-collector-contrib/exporter/prometheusremotewriteexporter v0.104.0
 	github.com/open-telemetry/opentelemetry-collector-contrib/internal/common v0.104.0
 	github.com/open-telemetry/opentelemetry-collector-contrib/pkg/pdatautil v0.104.0

receiver/prometheusreceiver/metrics_receiver.go

@@ -8,18 +8,19 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"hash/fnv"
 	"io"
 	"net/http"
 	"net/url"
 	"os"
 	"reflect"
 	"regexp"
+	"sort"
 	"sync"
 	"time"
 	"unsafe"
 
 	"github.com/go-kit/log"
-	"github.com/mitchellh/hashstructure/v2"
 	"github.com/prometheus/client_golang/prometheus"
 	commonconfig "github.com/prometheus/common/config"
 	"github.com/prometheus/common/model"
@@ -145,6 +146,33 @@ func (r *pReceiver) startTargetAllocator(allocConf *TargetAllocator, baseCfg *Pr
 	return nil
 }
 
+// Calculate a hash for a scrape config map.
+// This is done by marshaling to YAML because it's the most straightforward and doesn't run into problems with unexported fields.
+func getScrapeConfigHash(jobToScrapeConfig map[string]*config.ScrapeConfig) (uint64, error) {
+	var err error
+	hash := fnv.New64()
+	yamlEncoder := yaml.NewEncoder(hash)
+
+	jobKeys := make([]string, 0, len(jobToScrapeConfig))
+	for jobName := range jobToScrapeConfig {
+		jobKeys = append(jobKeys, jobName)
+	}
+	sort.Strings(jobKeys)
+
+	for _, jobName := range jobKeys {
+		_, err = hash.Write([]byte(jobName))
+		if err != nil {
+			return 0, err
+		}
+		err = yamlEncoder.Encode(jobToScrapeConfig[jobName])
+		if err != nil {
+			return 0, err
+		}
+	}
+	yamlEncoder.Close()
+	return hash.Sum64(), err
+}
+
 // syncTargetAllocator request jobs from targetAllocator and update underlying receiver, if the response does not match the provided compareHash.
 // baseDiscoveryCfg can be used to provide additional ScrapeConfigs which will be added to the retrieved jobs.
 func (r *pReceiver) syncTargetAllocator(compareHash uint64, allocConf *TargetAllocator, baseCfg *PromConfig) (uint64, error) {
@@ -155,7 +183,7 @@ func (r *pReceiver) syncTargetAllocator(compareHash uint64, allocConf *TargetAll
 		return 0, err
 	}
 
-	hash, err := hashstructure.Hash(scrapeConfigsResponse, hashstructure.FormatV2, nil)
+	hash, err := getScrapeConfigHash(scrapeConfigsResponse)
 	if err != nil {
 		r.settings.Logger.Error("Failed to hash job list", zap.Error(err))
 		return 0, err