Extract consumer group member_count as a separate metric

Added a new consumer_group_members topic to capture number of members
in a consumer group and removed the `member_count` label from consumer_group_info

Fixes #103

prometheus/collect_consumer_groups.go

@@ -2,10 +2,11 @@ package prometheus
 
 import (
 	"context"
+	"strconv"
+
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/twmb/franz-go/pkg/kerr"
 	"go.uber.org/zap"
-	"strconv"
 )
 
 func (e *Exporter) collectConsumerGroups(ctx context.Context, ch chan<- prometheus.Metric) bool {
@@ -43,12 +44,20 @@ func (e *Exporter) collectConsumerGroups(ctx context.Context, ch chan<- promethe
 				prometheus.GaugeValue,
 				float64(state),
 				group.Group,
-				strconv.Itoa(len(group.Members)),
 				group.Protocol,
 				group.ProtocolType,
 				group.State,
 				strconv.FormatInt(int64(coordinator), 10),
 			)
+			// total number of members in consumer groups
+			if len(group.Members) > 0 {
+				ch <- prometheus.MustNewConstMetric(
+					e.consumerGroupMembers,
+					prometheus.GaugeValue,
+					float64(len(group.Members)),
+					group.Group,
+				)
+			}
 		}
 	}
 	return true

prometheus/exporter.go

@@ -40,6 +40,7 @@ type Exporter struct {
 
 	// Consumer Groups
 	consumerGroupInfo              *prometheus.Desc
+	consumerGroupMembers           *prometheus.Desc
 	consumerGroupTopicOffsetSum    *prometheus.Desc
 	consumerGroupTopicPartitionLag *prometheus.Desc
 	consumerGroupTopicLag          *prometheus.Desc
@@ -144,7 +145,14 @@ func (e *Exporter) InitializeMetrics() {
 	e.consumerGroupInfo = prometheus.NewDesc(
 		prometheus.BuildFQName(e.cfg.Namespace, "kafka", "consumer_group_info"),
 		"Consumer Group info metrics. It will report 1 if the group is in the stable state, otherwise 0.",
-		[]string{"group_id", "member_count", "protocol", "protocol_type", "state", "coordinator_id"},
+		[]string{"group_id", "protocol", "protocol_type", "state", "coordinator_id"},
+		nil,
+	)
+	// Group Members
+	e.consumerGroupMembers = prometheus.NewDesc(
+		prometheus.BuildFQName(e.cfg.Namespace, "kafka", "consumer_group_members"),
+		"Consumer Group member count metrics. It will report the number of members in the consumer group",
+		[]string{"group_id"},
 		nil,
 	)
 	// Topic / Partition Offset Sum (useful for calculating the consumed messages / sec on a topic)