Add an observation behavior for logging, duration, and tracing

[kzu]

Summary

Ship one observation behavior that logs the call, records a System.Diagnostics.Metrics duration, and writes an Activity. Stop the timer when a returned Task or ValueTask completes, using Proceed.

API

The behavior takes an ILogger, the meter or instrument to update, and an optional redaction callback. The callback sees the invocation before any argument value is logged or tagged and can replace those values. Tags include the stunt type and the member name.

Behavior

• Observation wraps the rest of the pipeline: one activity and one duration cover every retry attempt.
• Run redaction before any argument is written.
• On a synchronous exception, record the failure and the duration, then let the exception propagate.
• On an awaitable, return it immediately and stop the timer when it completes, including fault and cancellation.

Acceptance

• A completed call records one duration and one activity named for the member.
• A redaction callback can drop an argument before it is logged.
• The duration of a Task<T> includes the time until that task completes.

Composition

Outer to inner: observation, resilience, transaction, cache, target.