Added default first hist bucket if needed

[yonatang]

Background

Fixes #522

When reporting hist with buckets that doesn't start with 0, any requests that won't fit the first bucket will fall into the last bucket, skewing the results. Please see discussion in issue #522 .

This PR will check if the first bucket starts with 0. If not, it will create a default bucket for such cases. For example, this input
-hist[5ms,10ms,1s] will generate the following buckets: [0,5ms], [5ms,10ms], [10ms,1s], [1s,+Inf] (and not [5ms,10ms], [10ms,1s], [1s,+Inf])

Checklist

• Git commit messages conform to community standards.
• Each Git commit represents meaningful milestones or atomic units of work.
• Changed or added code is covered by appropriate tests.