*: add metrics to prometheus

[djshow832]

What problem does this PR solve?

Issue Number: close None

Problem Summary:
Add metrics to prometheus. This doesn't include grafana.

What is changed and how it works:
Add metrics to prometheus.
StartMonitor and pushMetric are copied from TiDB. pushMetric pushes the metrics to the address specified in the configurations.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Open 3080/api/metrics and you'll see:

# HELP weirproxy_balance_b_conn Number of backend connections.
# TYPE weirproxy_balance_b_conn gauge
weirproxy_balance_b_conn{backend="192.168.0.101:60582"} 1
# HELP weirproxy_balance_b_status Gauge of backend status.
# TYPE weirproxy_balance_b_status gauge
weirproxy_balance_b_status{backend="192.168.0.101:60582",status="down"} 0
weirproxy_balance_b_status{backend="192.168.0.101:60582",status="healthy"} 1
# HELP weirproxy_monitor_keep_alive_total Counter of proxy keep alive.
# TYPE weirproxy_monitor_keep_alive_total counter
weirproxy_monitor_keep_alive_total 2
# HELP weirproxy_monitor_time_jump_back_total Counter of system time jumps backward.
# TYPE weirproxy_monitor_time_jump_back_total counter
weirproxy_monitor_time_jump_back_total 0
# HELP weirproxy_server_connections Number of connections.
# TYPE weirproxy_server_connections gauge
weirproxy_server_connections 1
# HELP weirproxy_session_query_duration_seconds Bucketed histogram of processing time (s) of handled queries.
# TYPE weirproxy_session_query_duration_seconds histogram
weirproxy_session_query_duration_seconds_bucket{backend="192.168.0.101:60582",cmd_type="Query",le="0.0005"} 0
weirproxy_session_query_duration_seconds_bucket{backend="192.168.0.101:60582",cmd_type="Query",le="0.001"} 1
weirproxy_session_query_duration_seconds_bucket{backend="192.168.0.101:60582",cmd_type="Query",le="0.002"} 1
weirproxy_session_query_duration_seconds_bucket{backend="192.168.0.101:60582",cmd_type="Query",le="0.004"} 1

Release note

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None