feat: enabled multiple metrics per ref

[calmez]

Context

We have pipelines running on the same git ref but with different settings in variables.
Before this change here we had the problem that we were missing metrics for some of these runs.

What changed

We generalized how pipelines are cached using the Store instead of the LastPipeline on the Ref object.
Furthermore, we used variables (if configured) for deciding if a new metric needs to be created or not.

We have tried this approach and can confirm that this now solves the issue we had before.

Backwards compatibility

The new behavior is backwards compatible. In the pipelines section of a pull section in the config there's a new key per_ref that you can use to set how many most recent pipelines you want to fetch per ref. The default is set to 1 which should result in the same behavior as before this change. That means that existing users will not be affected by this and have to actively configure this behavior.

[calmez]

@mvisonneau Any chance we can get this up in mainline?
We're using this already in our deployment and it seems quite stable and certainly very useful for us.