Remove the zero bucket boundary from http.server.request.duration and http.client.request.duration

[trask]

Fixes #298

Changes

Removes the zero bucket boundary from http.server.request.duration and http.client.request.duration.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• CHANGELOG.md updated for non-trivial changes.
• schema-next.yaml updated with changes to existing conventions.

[JamesNK]

After this change, it's odd that the default buckets - https://github.com/open-telemetry/opentelemetry-specification/blob/7a7b36e88742f834c1097cc7f1c1d13182cb1142/specification/metrics/sdk.md#explicit-bucket-histogram-aggregation - have zero, and this one doesn't.

Also, the number of buckets is now inconsistent between these durations and the defaults. It was easy to reason that these buckets were simply the defaults but adjusted to seconds.

[trask]

my interpretation of the default buckets is that they are (sadly) just a legacy artifact, and are pretty meaningless now that the default unit has changed to seconds