improve metrics to help better debug

[lance6716]

Feature Request

Is your feature request related to a problem? Please describe:

Describe the feature you'd like:

• fix replicationLagGauge syncer: add metric replicationLagGauge back #1709
• current DML QPS
• file size of upstream binlog / DM comsuming
• batch of a job in DML sync queue
• explain every step in dispatchBinlogDurationHistogram: resolve causality + block on sending to DML sync queue + save table checkpoint
• each DML size / number in a binlog event
• calculate a QPS upper bound by #connection * 1 / SQL time cost

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[Ehco1996]

/assign Ehco1996