Add requester field and enricher hook to controller metrics

[DylanBlakemore]

Summary

• Add requester field to Zexbox.Metrics.ControllerSeries (not populated by default)
• Add Zexbox.Metrics.ControllerSeriesEnricher behaviour and an :enricher option on Zexbox.Metrics.start_link/1 (also configurable via config :zexbox, :metrics_enricher)
• Invoke the enricher from the metric handler with exception fallback

Details

Host apps frequently need to attach context to the controller metric that zexbox itself doesn't know about — most immediately, an identifier for the caller derived from an API key lookup. Rather than baking specific knowledge of those values into zexbox, this adds a small extension point: a Plug-shaped behaviour (init/1 + call/3) that receives the in-flight ControllerSeries and the conn, and returns a possibly-modified series. The host app configures their enricher module either at supervision time or via application env, and uses it to copy values they've already stashed on conn.assigns (e.g. an API key description set by their auth plug) onto the series.

The new requester field is the canonical place to put that caller identifier, but the enricher is free to set any tag or field on the series. Enricher exceptions are caught by the metric handler and logged — a buggy enricher will not break telemetry; the un-enriched series is written instead.