Fix Prometheus promtool errors

Checking the Prometheus metrics with promtool check metrics returns several errors:

gitaly_authentications counter metrics should have "_total" suffix
gitaly_service_client_requests counter metrics should have "_total" suffix
gitaly_smarthttp_deepen_count counter metrics should have "_total" suffix
gitaly_smarthttp_deepen_count non-histogram and non-summary metrics should not have "_count" suffix
gitaly_spawn_timeout_count counter metrics should have "_total" suffix
gitaly_spawn_timeout_count non-histogram and non-summary metrics should not have "_count" suffix
gitaly_supervisor_health_checks counter metrics should have "_total" suffix