MM-39215: Add DBStatsCollector to metrics (#21957)

https://mattermost.atlassian.net/browse/MM-39215

```release-note
We now have Grafana metrics for DB connection metrics.
They are:
max_open_connections
open_connections
in_use_connections
idle_connections
wait_count_total
wait_duration_seconds_total
max_idle_closed_total
max_idle_time_closed_total
max_lifetime_closed_total
```

app/platform/service_test.go

@@ -141,6 +141,7 @@ func TestMetrics(t *testing.T) {
 		mockMetricsImpl := &mocks.MetricsInterface{}
 		mockMetricsImpl.On("Register").Return()
 		mockMetricsImpl.On("ObserveStoreMethodDuration", mock.Anything, mock.Anything, mock.Anything).Return()
+		mockMetricsImpl.On("RegisterDBCollector", mock.AnythingOfType("*sql.DB"), "master")
 
 		th := Setup(t, StartMetrics(), func(ps *PlatformService) error {
 			ps.metricsIFace = mockMetricsImpl