Update metrics docs
**FamilyName**: service_center
### Server
-1. http_request_total
-1. http_success_total
-1. http_request_durations_microseconds
-1. http_query_per_seconds
+1. **http_request_total**: The total number of received service requests.
+1. **http_success_total**: Total number of requests responding to status code 2xx or 3xx.
+1. **http_request_durations_microseconds**: The latency of http requests.
+1. **http_query_per_seconds**: TPS of http requests.
### Pub/Sub
-1. notify_publish_total
-1. notify_publish_durations_microseconds
-1. notify_pending_total
-1. notify_pending_durations_microseconds
-1. notify_subscriber_total
+1. **notify_publish_total**: The total number of instance events.
+1. **notify_publish_durations_microseconds**: The latency between the event generated in ServiceCenter and received by the client.
+1. **notify_pending_total**: The total number of pending instances events.
+1. **notify_pending_durations_microseconds**: The latency of pending instances events.
+1. **notify_subscriber_total**: The total number of subscriber, e.g. Websocket, gRPC.
### Meta
-1. db_heartbeat_total
-1. db_heartbeat_durations_microseconds
-1. db_service_total
-1. db_domain_total
-1. db_instance_total
-1. db_schema_total
-1. db_framework_total
+1. **db_heartbeat_total**: The total number of received instance heartbeats.
+1. **db_heartbeat_durations_microseconds**: The latency of received instance heartbeats.
+1. **db_service_total**: The total number of micro-services.
+1. **db_domain_total**: The total number of domains.
+1. **db_instance_total**: The total number of instances.
+1. **db_schema_total**: The total number of schemas.
+1. **db_framework_total**: The total number of SDK frameworks.
### Backend
-1. db_backend_event_total
-1. db_backend_event_durations_microseconds
-1. db_dispatch_event_total
-1. db_dispatch_event_durations_microseconds
-1. db_backend_operation_total
-1. db_backend_operation_durations_microseconds
-1. db_backend_total
+1. **db_backend_event_total**: The total number of received backend events, e.g. etcd, Mongo.
+1. **db_backend_event_durations_microseconds**: The latency between received backend events and finish to build cache.
+1. **db_dispatch_event_total**: The total number of dispatch events to resource handlers.
+1. **db_dispatch_event_durations_microseconds**: The latency between received backend events and finish to dispatch.
+1. **db_backend_operation_total**: The total number of backend requests, e.g. etcd, mongo.
+1. **db_backend_operation_durations_microseconds**: The latency of backend requests.
+1. **db_backend_total**: The total number of backend instances.
### System
-1. db_sc_total
+1. **db_sc_total**: The total number of ServiceCenter instances.
1. process_resident_memory_bytes
1. process_cpu_seconds_total
1. process_cpu_usage
