Replace event.metric_names with metric_names_hash

A single GCP metric name can be quite long, for example:

  subscription.streaming_pull_mod_ack_deadline_message_operation.count

we may collect enough metrics to overflow the current
length limit for dimension fields.

We hash the metric names value using SHA-256 to have a constant length
value.

I'm not 100% sure SHA-256 is the best option for this use case; we
don't have cryptographic solid needs, so we can probably use a
simpler algorithm to save computing cycles while getting a shorter hash
value.

We also drop `event.metric_names` because is no longer needed.

metricbeat/docs/fields.asciidoc

@@ -35303,10 +35303,10 @@ GCP module
 
 
 
-*`event.metric_names`*::
+*`event.metric_names_hash`*::
 +
 --
-The comma-separated list metric names collected in the batch. For example, l3.external.ingress_packets.count,l3.external.ingress.bytes. Required to support TSDB.
+The SHA-256 hash of the comma-separated list metric names collected in the batch. For example, l3.external.ingress_packets.count,l3.external.ingress.bytes > 24848afd85b65b9e168fa5dee12bd3e121c58a504cc0b4386a15bd9bcd065a5d. Required to support TSDB.
 
 
 type: keyword

x-pack/metricbeat/module/gcp/_meta/fields.yml

@@ -7,10 +7,10 @@
     - name: event
       type: group
       fields:
-        - name: metric_names
+        - name: metric_names_hash
           type: keyword
           description: >
-            The comma-separated list metric names collected in the batch. For example, l3.external.ingress_packets.count,l3.external.ingress.bytes. Required to support TSDB.
+            The SHA-256 hash of the comma-separated list metric names collected in the batch. For example, l3.external.ingress_packets.count,l3.external.ingress.bytes > 24848afd85b65b9e168fa5dee12bd3e121c58a504cc0b4386a15bd9bcd065a5d. Required to support TSDB.
     - name: gcp
       type: group
       fields:

x-pack/metricbeat/module/gcp/metrics/timeseries.go

@@ -6,6 +6,8 @@ package metrics
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"fmt"
 	"strings"
 
@@ -140,7 +142,12 @@ func createEventsFromGroups(service string, groups map[string][]KeyValuePoint) [
 			event.RootFields = group[0].ECS
 		}
 
-		_, _ = event.RootFields.Put("event.metric_names", strings.Join(metricNames, ","))
+		// Hashes metric names string using SHA-256 to always have
+		// a constant length value and avoid overflowing the
+		// current TSDB dimension field limit (1024).
+		metricNamesHash := hash(strings.Join(metricNames, ","))
+
+		_, _ = event.RootFields.Put("event.metric_names_hash", metricNamesHash)
 
 		events = append(events, event)
 	}
@@ -188,3 +195,10 @@ func (m *MetricSet) groupTimeSeries(ctx context.Context, timeSeries []timeSeries
 
 	return groupedMetrics
 }
+
+// hash return the SHA-256 hash of the input string.
+func hash(s string) string {
+	h := sha256.New()
+	h.Write([]byte(s))
+	return hex.EncodeToString(h.Sum(nil))
+}

x-pack/metricbeat/module/gcp/metrics/timeseries_test.go

@@ -475,7 +475,7 @@ func TestCreateEventsFromGroup(t *testing.T) {
 					"cloud.provider":          "gcp",
 					"cloud.region":            "us-west",
 					"event": mapstr.M{
-						"metric_names": "metric1,metric2",
+						"metric_names_hash": "2a990d0ce61c177d3bc05def9f536de164d3ea10056b7120414ad370a0f686c0",
 					},
 				},
 			},
@@ -504,7 +504,7 @@ func TestCreateEventsFromGroup(t *testing.T) {
 					"cloud.provider":          "gcp",
 					"cloud.region":            "us-east",
 					"event": mapstr.M{
-						"metric_names": "metric3,metric4",
+						"metric_names_hash": "f7c8d8a9a75411effbd2171a8fa984a5e618eeca3911829b03c7e3e3d6e74522",
 					},
 				},
 			},
@@ -532,7 +532,7 @@ func TestCreateEventsFromGroup(t *testing.T) {
 					"cloud.provider":          "gcp",
 					"cloud.region":            "us-east",
 					"event": mapstr.M{
-						"metric_names": "metric5",
+						"metric_names_hash": "ce7dee36d9ab56de52176bf05d21c0bc446da9a3134727bf5e300604f2ca63ef",
 					},
 				},
 			},