Add single-scope observable metric dispatcher

[fralalonde]

For libs and modules that cannot predict where metrics should be sent. Send to AppMetrics and let the actual app wire its observers upon start.

[fralalonde]

In progress, as dispatch module. Adds global handler hook for late-defined metrics (i.e. from soft config).

Thinking about adding dispatch_metrics! and aggregate_metrics! macros.

[fralalonde]

fixed with Proxy