Add min/max fields to HistogramDataPoint

[jmacd]

Adds a definition for min and max in the Histogram data point.
See the definition of these fields in the data model.
Fixes #266.

[jmacd]

Note the idea would be to apply this to the Histogram point after PR #272.

[jmacd]

This is stale and we may revisit this topic after other histogram work is finished.

[jmacd]

I modified this PR to use proto3 "optional" which means to use explicit presence, see this document:
https://chromium.googlesource.com/external/github.com/protocolbuffers/protobuf/+/refs/heads/master/docs/field_presence.md

[mx-psi]

Now that #322 has been merged, should this PR add min/max fields for ExponentialHistogram too?

[jmacd]

The build says:

opentelemetry/proto/metrics/v1/metrics.proto: This file contains proto3 optional fields, but --experimental_allow_proto3_optional was not set.

was fixed

[codeboten]

I believe with open-telemetry/opentelemetry-collector#4929 this is now fully unblocked. @jmacd please rebase the branch

[jmacd]

Thank you @codeboten!

[codeboten]

@bogdandrutu please review

[bogdandrutu]

@jmacd @codeboten as in any ideal world, we should start with fixing a bug first #336 than adding new functionality. Should we merge the optional sum?

[codeboten]

Either one is fine with me @bogdandrutu, I don't think I have permissions to push changes to #336 to unblock the PR 😬

[bogdandrutu]

Let’s fix that first. Maybe clone that PR and open a new one with necessary changes.

[codeboten]

@bogdandrutu see #366

[bogdandrutu]

@codeboten I want to do a release #367 then start using this in collector to prove everything is fine, then will merge this.

[Falmarri]

I modified this PR to use proto3 "optional" which means to use explicit presence, see this document: https://chromium.googlesource.com/external/github.com/protocolbuffers/protobuf/+/refs/heads/master/docs/field_presence.md

This is really annoying. It means opentelemetry isn't build-able or usable on ubuntu bionic with upstream provided packages.