Fix tally timer metric

[yycptt]

What changed?

• Switch back to tally.Timer instead of histogram for timer metrics. The reason is that when using histogram.RecordDuration, if the duration is higher than the max bucket defined, an Inf value will be added to the sum metric emitted and breaks the average calculation. When using histogram.RecordValue, if value is higher than the max bucket defined, no metric will be emitted for the sum metric, and make it inaccurate. Also for RecordValue, the value added to the sum metric is always the defined bucket boundary instead of the actual metric value.
• tally.Timer implementation has none of the problems mentioned above.
• However there's no when to specify the buckets when emitting tally.Timer metric, and the default reporter bucket will be used. So wired up the code to specify the default bucket for reporter, which is only used by Timer metric.
• Bring back the code path for DefaultHistogramBuckets and DefaultHistogramBoundaries so that the code is backward compatible with older versions. Since value defined in ClientConfig.PerUnitHistogramBoundaries was not previously used, it will only be used when neither DefaultHistogramBuckets nor DefaultHistogramBoundaries is specified.
• Also added more comments for DefaultHistogramBuckets and DefaultHistogramBoundaries config fields.

Why?

• Fix tally timer metric

How did you test it?

• Tested locally.

Potential risks
No risks.

Is hotfix candidate?

• Yes.