Don't collect empty metrics from storage

[spenczar]

Description

Without doing this, the pb_metric2.Metric has no value for the oneof data field, which means it does not have any known type. This breaks many exporters. Instead, when faced with no data points for a metric, we should present that empty dataset.

Fixes #3277.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

I have tested this a little with tox, but I haven't written thorough tests. I'm not sure where I'd put those - this is a big, complicated repo, and writing good tests is a challenge.

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: spenczar / name: Spencer Nelson (056d3bb)
• ✅ login: ocelotl / name: Diego Hurtado (cf0492b)

[srikanthccv]

This has some overlap with this PR #3335. I think the concerns expressed are valid. @aabmass was there any SIG discussion on it?

[aabmass]

Let's discuss it in next week's SIG. I agree there is some overlap here.

[aabmass]

I think this is an SDK bug and the exporter shouldn't have to handle the special case. Lets see if we can fix it in the SDK instead.

[ocelotl]

@aabmass brought this topic in the SIG, please leave here any opinions you may have.

[ocelotl]

Superseded by #3335, closing