MM-22786 enterprise metrics for logging (#15313)

Adds metrics for logging engine which are collected by Prometheus.
- current queue level(s)
- rate of logging records emitted
- rate of logging errors

einterfaces/metrics.go

@@ -3,6 +3,8 @@
 
 package einterfaces
 
+import "github.com/mattermost/logr"
+
 type MetricsInterface interface {
 	StartServer()
 	StopServer()
@@ -56,4 +58,6 @@ type MetricsInterface interface {
 	ObservePluginMultiHookIterationDuration(pluginID string, elapsed float64)
 	ObservePluginMultiHookDuration(elapsed float64)
 	ObservePluginApiDuration(pluginID, apiName string, success bool, elapsed float64)
+
+	GetLoggerMetricsCollector() logr.MetricsCollector
 }