header_rewrite: count operators executed and conditions evaluated

[moonchen]

A transaction's header_rewrite CPU scales with how many operators it runs and
conditions it evaluates, which is set by the configured rules -- the per-hook
invocation count alone cannot distinguish a small ruleset from a large one. Add
proxy.process.plugin.header_rewrite.{operators,conditions}, incremented in the
inline Operator::do_exec and Condition::do_eval chain walkers. do_eval respects
short-circuit, so only conditions actually evaluated are counted. The counters
are created once, from both TSPluginInit and TSRemapInit.

[Copilot]

Pull request overview

This PR adds process-wide “work” metrics to the header_rewrite plugin to better model CPU cost per transaction by counting operators executed and conditions evaluated, rather than relying only on per-hook invocation counts.

Changes:

• Add new metrics proxy.process.plugin.header_rewrite.operators and .conditions, created once and incremented in the hot-path chain walkers (Operator::do_exec, Condition::do_eval).
• Initialize the metrics from both global plugin init (TSPluginInit) and remap init (TSRemapInit) so global + remap usage share the same process stat IDs.
• Add a gold test plus a minimal rules file to validate the metrics are present and increasing.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/gold_tests/pluginTest/header_rewrite/rules/metrics.conf	Adds a simple ruleset that triggers known operator/condition activity for metric validation.
tests/gold_tests/pluginTest/header_rewrite/header_rewrite_metrics.test.py	Adds a gold test that drives traffic and queries the new metrics via traffic_ctl.
plugins/header_rewrite/statement.h	Declares global stat IDs and init_hrw_work_stats() for the new metrics.
plugins/header_rewrite/statement.cc	Defines stat IDs and implements one-time creation via TSStatCreate.
plugins/header_rewrite/operator.h	Increments the operators-executed metric once per operator execution in do_exec().
plugins/header_rewrite/condition.h	Increments the conditions-evaluated metric once per evaluated condition in do_eval() (short-circuit respected).
plugins/header_rewrite/header_rewrite.cc	Calls init_hrw_work_stats() from both TSPluginInit and TSRemapInit.