Add txn write conflict metrics

[ImSjt]

Issue Number: close #1550
Add transaction write conflict metrics.

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[ImSjt]

/approve zyguan

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[ekexium]

LGTM. Please resolve the conflicts

[ImSjt]

@ekexium PTAL again.

[ekexium]

Actually I think the metric is not related to lock_resolver. It's more like "WriteConflicts errrors generated from the client side".
Maybe we can remove LockResolverCountWithWriteConflict and make this one a new metric. What do you think?

[ImSjt]

@ekexium It makes sense. I think we can add a metrics to count the error results of the doActionOnMutations function, so that we can clearly know the final result of the different actions in the commit phase. What do you think?

TikvCommitterErrorMetrics = prometheus.NewCounterVec(
    prometheus.CounterOpts{
      Namespace: namespace,
      Subsystem: subsystem,
      Name:      "committer_err_total",
      Help:      "Counter of committer errors.",
    }, []string{LblType, LblResult})

LblType: prewrite, commit, cleanup...
LblResult: error.Error()

[ekexium]

Have you seen real needs for such metrics? Since metrics are not free, I suggest we either add just one metric for write conflicts or remove the existing one, unless we can justify their value.