Metrics redesign: master issue

[tobz]

This is the master issue for tracking the larger Metrics Redesign (tm).

At a high level, there are a few things we want to do or consider:

• defining metric metadata (a la Prometheus' HELP text) (Metrics redesign: defining metric metadata #68)
• defining metrics via macros (tracing_core::Callsite but for metrics) (Metrics redesign: defining metrics via macros #69)
• labels via context (bring tracing-error to `metrics) (Metrics redesign: labels via context #70)
• dissolve metrics-runtime into its individual parts to support reuse
• eliminate the notion of exporters and observers, and elevate metrics

[tobz]

At this point, all of this work is done, albeit only available on alpha releases and the main branch.

We'll be working to polish up available layers, exporters, examples, and documentation as we work towards a 1.0 release.