Investigate buckets support for Tally Timer

[objectiser]

Requirement - what kind of business use case are you trying to solve?

With #63 it is now possible to define Histograms with buckets - but also define duration based buckets for the Timer metric type.

Problem - what in Jaeger blocks you from solving the requirement?

The buckets are being used with the Tally Histogram type - but it appears the Tally Timer type does not enable buckets to be defined.

Proposal - what do you suggest to solve the problem or improve the existing situation?

Either buckets will have to be ignored for Tally impl (with a warning being produced?), or the underlying representation should be changed to a Tally Histogram with DurationBuckets.