src/metrics/family.rs: Add remove and clear methods, and tests

[phyber]

This PR adds Family::remove as discussed a little in #36 along with a basic test for the behaviour. We also add a Family::clear method as discussed in this PR.

The Family::remove method returns a bool to indicate if a label set was removed, which matches what the Go client does when removing its equivalent of a label set.

The Family::clear method does not return anything and always successfully clears the label sets.

[mxinden]

Solid work! Thanks, especially for the doc comment and example!

According to the client library documentation there should be both a remove and a clear. I don't have a strong opinion for the latter. But given that is should be easy to add, what do you think of including it in this pull request?

Metrics with labels SHOULD support a remove() method with the same signature as labels() that will remove a Child from the metric no longer exporting it, and a clear() method that removes all Children from the metric. These invalidate caching of Children.

https://prometheus.io/docs/instrumenting/writing_clientlibs/#labels

[phyber]

I'd be happy to work on adding a clear method here as well. I might not have that before the end of this week though, as I'm away for a few days :)

[mxinden]

No rush @phyber. Thanks for the help.

[phyber]

Family::remove_label_sets has now been renamed to Family::remove, and a Family::clear method has been added.

The redundant Family::write private method was removed.

[mxinden]

Thanks for the follow-ups!

Just needs a bump of the patch version in Cargo.toml and a changelog entry in CHANGELOG.md.

[phyber]

Rebased onto the current master and added entries to the CHANGELOG for this PR. I tried to use the same type of language in the CHANGELOG that was used the last time new methods were added to Family.

I didn’t bump the version in Cargo.toml since I guess this will all be released under the (currently unreleased) v0.19.0.

[mxinden]

🙏