feat_jc_rhai_duration_metric.md

Emit apollo.router.operations.rhai.duration histogram metric for Rhai script callbacks

A new apollo.router.operations.rhai.duration histogram metric (unit: s, value type: f64) is now emitted for every Rhai script callback execution across all pipeline stages. This mirrors the existing apollo.router.operations.coprocessor.duration metric.

Attributes on each datapoint:

• rhai.stage — the pipeline stage (e.g. RouterRequest, SubgraphResponse)
• rhai.succeeded — true if the callback returned without throwing

By @theJC in #9072