Observability (o11y) helpers for the NEAR codebase.

This crate contains all sorts of utilities to enable a more convenient observability implementation in the NEAR codebase.

The are three infrastructures:

• tracing, for structured, hierarchical logging of events (see [default_subscriber] function function in particular)
• metrics -- convenience wrappers around prometheus metric, for reporting statistics.
• io-tracer -- custom infrastructure for observing DB accesses in particular (mostly for parameter estimator)