Should we have metrics registry at some point?

[lmolkova]

Metrics should be referenceable in the same way as attributes are, here're some examples:

• Add .NET metrics #283 introduces a bunch of .NET-specific metrics, but also refers to standard HTTP ones
• specific messaging system/db may want to specify which generic metrics it reports and add extra attributes, etc
• things like connection-level metrics can be shared between other semconvs Common socket connection conventions #454

Referencing mechanism for metrics would be useful to:

• specify differences in a formal manner (e.g. additional attributes, opt-in attributes that's never reported, etc)
• codegen that can generate code for specific metric