HA: add and refactor metrics

[csuzhangxc]

Feature Request

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

Describe the feature you'd like:

• add more metrics about HA, like metrics in DM-master.
• refactor previous metrics for HA (like updating instance to source for metrics in DM-worker).

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[lance6716]

part of new metrics[design]:

• state of workers master: add metrics for worker state #776
• state of subtasks now contains label "source-id" worker: add metrics #772
• number of operate errors (of different type?) by worker, so we could see worker should be scheduled out. worker: add metrics #772

[lance6716]

part of new metrics:

• DDL errors will be reported in master: add metrics for handle DDL #790, but usage of these metrics is not very good
• going to report DDL number in un-synced/un-resolve states master: add metric indicate number of pending DDL state (pessimistic only) #791