Allow CallsiteParameterAdder to use custom event dict keys

[pranjalm37]

Summary

Closes #553.

CallsiteParameterAdder.parameters currently only accepts a Collection[CallsiteParameter], and each parameter is always keyed in the event dict by its own CallsiteParameter.value (e.g. "lineno", "func_name"). There's no way to use a different key without a separate renaming processor.

This implements exactly the API proposed in the issue: parameters now also accepts a Mapping[str, CallsiteParameter]. When a mapping is passed, its keys become the event dict keys instead of the default enum values — useful for conforming to a third-party log ingest pipeline's expected field names (the issue mentions ECS and Datadog specifically) without a second processor.

CallsiteParameterAdder({"tid": CallsiteParameter.THREAD, "pid": CallsiteParameter.PROCESS})

Passing a plain collection — all existing usage — is completely unaffected: each parameter is still keyed by its own enum value, exactly as before.

Note for maintainer

I know this is a feature request rather than a bug fix, and the API shape here was proposed by the original reporter, not something I've gotten sign-off on. I implemented it as described since it looked genuinely small, useful, and backwards-compatible, but I'm very open to a different shape (or to this not being the direction you want at all) — happy to adjust or close if so.

Implementation

• _active_handlers now stores (key, handler) tuples instead of (CallsiteParameter, handler). key is either the mapping's key (custom) or parameter.value (default), computed once in __init__.
• Both code paths in __call__ — the structlog-native path (_active_handlers) and the foreign-LogRecord path (_record_mappings) — use the resolved key consistently, so custom keys work whether the event originated from structlog or from stdlib logging.

Test plan

• test_custom_keys_structlog_originated — mapping keys show up correctly for structlog-native calls.
• test_custom_keys_foreign_log_record — same, for the stdlib-LogRecord-derived path.
• test_custom_keys_pickleable — a mapping-configured instance is still picklable (the class is explicitly designed to be, for propagating config to subprocesses).
• Full suite: pytest tests/ → 903 passed (up from 900 pre-change), 21 skipped.
• mypy src/structlog/processors.py clean.
• ruff check / ruff format --check clean.
• Added a changelog entry under ## [Unreleased].

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks

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_{Comparing pranjalm37:feat/callsite-parameter-adder-rename-keys-553 (54a85ac) with main (bf3cfd0)¹}

Footnotes

1. No successful run was found on main (ab24a26) during the generation of this report, so bf3cfd0 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩