metrics: Meta issue about Data Model Stability Guarantees

[hdost]

There are a couple of issues which are looking at breaking changes.

According to the README.md right now. Metrics is considered Beta. However, as part of the 0.5.0 release there is a mention:

This is a Release Candidate to declare Metrics part of the protocol Stable.

I believe from what I understand that it should still be considered Beta given it's state. And the fact that only as of 0.7.0 was it even marked as Beta

Related Change Propositions:
#257
#264
#272

As mentioned in #264 (comment) we should try to make the transition smooth.

Taking a cursory look at the release history it looks like the last breaking change release was made 2020-08-31 in the form of release 0.5.0. So according to the maturity matrix we are free to make a release which is not backwards compatible for metrics when we want.

My proposal would be:

1. Add target types and deprecations to the the protocol
2. Release the "compatible" pre-release version 0.8.0.
3. Incorporate the incompatible changes to the protocol.
4. Release the "incompatible" version whether that be 0.9.0 or which ever. And this could be within a few days of each other possibly.

This change would also model for metrics to actually be Stable according to the maturity matrix.

If something like this doesn't occur there is a strong risk of slippage on the whole metric roadmap. Which wouldn't necessarily be the end of the world, but estimates might need to be re-considered.

[hdost]

I also discovered (#269) PR which changes the name of Summary. This is something that would techncially break compatibility. Likely if we want to follow the above proposal we should consider reverting it before Step 2. Or more than likely re-add it and have the plain summary. The DoubleSummary should be marked deprecated, and then for the case of Step 3. it could then be removed.

[hdost]

cc: @jmacd , @bogdandrutu

[jmacd]

@hdost Acknowledging your point, we have to be careful how we describe these changes before the next release. Also we did not write a Changelog entry in #270, which led me back to this issue because I'm not sure how to classify the change. It's partly breaking (remove IntHistogram) and partly non-breaking (just a rename) -- because we have not declared JSON stability.

[hdost]

👍🏼 I do remember that JSON stability wasn't set, but I wasn't sure how things affect on the Protobuf side.

[tigrannajaryan]

I believe this issue is now outdated. Metrics are declared Stable.

Closing. Please open a new issue if you believe something else is needed.