Add per-plugin workload counters

[moonchen]

Add per-plugin metrics that allow us to track how much work each plugin is doing by counting their invocations, intercept bytes, and intercept transfers.

Add proxy.process.plugin..{invocations,bytes, transfers}, keyed by the plugin DSO basename and bounded by the number of loaded plugins.

Global plugins load via raw dlopen and previously carried no identity, so they are given a PluginThreadContext around TSPluginInit; the continuations they create then carry plugin identity the same way remap plugins already do.

[Copilot]

Pull request overview

This PR adds per-plugin workload metrics to Apache Traffic Server so operators can attribute plugin activity to specific plugin DSOs (by basename), covering callback invocations as well as PluginVC intercept transport bytes/transfers. It also adds a gold test to validate that the new counters increment for both global and remap plugins.

Changes:

• Add proxy.process.plugin.<name>.{invocations,bytes,transfers} counters and increment them from key dispatch / transport paths.
• Introduce a PluginThreadContext wrapper for global plugins so continuations they create carry plugin identity (similar to remap plugins).
• Add a gold test that drives traffic through a global plugin, a remap plugin, and a PluginVC intercept plugin and asserts the counters are non-zero.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/gold_tests/pluginTest/per_plugin_metrics/rules/global.conf	Adds a header_rewrite rule to ensure global-plugin continuation dispatch occurs per transaction.
tests/gold_tests/pluginTest/per_plugin_metrics/per_plugin_metrics.test.py	New gold test validating per-plugin invocation/bytes/transfers metrics via traffic_ctl metric get.
src/proxy/PluginVC.cc	Increments per-plugin bytes/transfers counters for PluginVC intercept data movement and captures counters at core allocation time.
include/proxy/PluginVC.h	Stores per-plugin counter pointers in PluginVCCore for intercept accounting.
src/proxy/Plugin.cc	Wraps global plugin initialization in a PluginThreadContext so created continuations carry plugin identity.
src/proxy/http/remap/RemapPlugins.cc	Counts remap plugin invocations at the remap dispatch site.
include/proxy/http/remap/PluginDso.h	Adds metric registration/increment helpers and per-plugin counter pointers to PluginThreadContext.
src/proxy/http/remap/PluginDso.cc	Implements metric-name tokenization and registers per-plugin counters on successful plugin load.
src/api/InkContInternal.cc	Counts per-plugin invocations for TSCont (continuation) callback dispatches.

[moonchen]

[approve ci]