Emit metrics related to binlog processing

[forge33]

Related issue: https://github.com/Shopify/schema-migrations/issues/5838

Description

This PR adds metrics around binlog processing so we can see what gh-ost is reading, forwarding, and filtering while streaming row events. Metrics follow similar patterns as backfilter's binlog tailer/reader.

Metric	Type	Tags	When
binlog_events	count	type:processed, table:<name>	Every rows event read from the binlog
binlog_rows	count	type:processed, table:<name>, binlog_event_type:insert|update|delete	Same
binlog_events	count	type:filtered, table:<name>	Rows event on a table that is not the migration table or changelog table
binlog_rows	count	type:filtered, table:<name>, binlog_event_type:...	Same
binlog_events	count	type:consumed, table:<name>, binlog_event_type:...	Rows event on the original table or _ghc changelog table
binlog_rows	count	type:consumed, table:<name>, binlog_event_type:...	Same
binlog_rows_in_event	histogram	table:<name>	Row count per consumed rows event
transaction_num_row_events	histogram	—	End of transaction: consumed row events on relevant tables (flush on next GTID, or on XID in file-position mode)
transaction_num_rows	histogram	—	End of transaction: logical rows in those consumed events
gtid_event_transaction_length_bytes	histogram	—	Each GTID event (TransactionLength, MySQL 8.0.2+)
unfiltered_commit_group_size	gauge	—	When LastCommitted changes (SequenceNumber - LastCommitted)
binlog_streamer_blocked_on_out_channel_ms	histogram	—	Time blocked sending a consumed rows event to the events channel

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.