cache_reporter design issue

[hannesm]

The Metrics.cache_reporter stores a map from metric sources to measurement. Now, the issue is that if you have a source that uses tags, and multiple measurements with different tags are used, only the latest one is stored. NB: each source has a uid, and comparison of a source created one way with the same parameters as another source are not identical.

A workaround is done e.g. in the mirage-net-solo5 package -- https://github.com/mirage/mirage-net-solo5/blob/v0.8.0/src/netif.ml#L82 -- where each new mirage-net-solo5 instance (value returned by Netif.connect) creates a fresh source.

It would be great to instead adapt the cache_reporter to properly cope with this. Eventually instead of a "Map.Make(Src)", there should be something that takes all tags into account (but here I'm lost how to write this down - esp. since 'a Tags.t is pretty tough (and there's no equality/comparison defined for Tags.t)...