Support histograms

[yurishkuro]

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

We've had situations where it would be useful to report histograms of non-timer values, e.g. a distribution of queue lengths, or the span sizes. Backends like Prometheus do support them.

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

The current metrics.Factory only allows timers that are internally implemented as histograms.

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

• Replace Factory.Timer() with Factory.Histogram(HistogramOptions{Name, Tags, Description, Buckets})
• Add static factory methods metrics.Timer(TimeOptions{HistogramOptions, Factory})

Any open questions to address

• Breaking change, may want to include in 2.x (#47)
• Decide if we want to use structs for arguments, to ensure better extensibility