feat(metasrv): add metrics for db_size

[everpcpc]

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Summary

add metrics

• metasrv_server_db_size

❯ curl localhost:28002/v1/metrics
# HELP metasrv_server_snapshot_key_num snapshot key numbers.
# TYPE metasrv_server_snapshot_key_num gauge
metasrv_server_snapshot_key_num 0
# HELP metasrv_server_db_size db size.
# TYPE metasrv_server_db_size gauge
metasrv_server_db_size 2102662

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is