Add missing operator metric

[mzhang77]

Feature Request

Is your feature request related to a problem? Please describe:
Some metrics registered here https://github.com/pingcap/tidb-operator/blob/master/pkg/metrics/metrics.go#L70 are not defined. Missing metrics are:

ReconcileTotal,
ReconcileErrors,
WorkerCount,

Describe the feature you'd like:
Define these missing metrics.

Describe alternatives you've considered:
NA

Teachability, Documentation, Adoption, Migration Strategy:
This issue is requested by a paid customer