NH-125600 Add ServiceEntrySpanProcessor._on_ending implementation to reduce overall name pool registrations

[tammy-baylis-swi]

Depends on #832.

Implements ServiceEntrySpanProcessor._on_ending, made available with OTel 1.40.0/0.61b0. This reduces total calls to pool.registered(transaction_name) from n + 1 to 1, uses the pool more efficiently (e.g. fewer intermediates taking spots). See ticket for more info.

Also includes accidental bugfix to 256 --> 255 char limit in integration tests.

API's set_transaction_name usage and behaviour are still the same; it will log fewer warning whenever pool capacity reached. Name setting and response time metrics attribute setting are also the same.

[Copilot]

Pull request overview

This PR updates the SolarWinds APM OpenTelemetry distribution to defer transaction-name pool registration until span ending by implementing ServiceEntrySpanProcessor._on_ending (available in newer OTel SDKs), reducing repeated pool.registered(...) calls and improving pool utilization. It also updates unit/integration tests to match the new “set attribute first, register at ending” behavior and corrects truncation expectations to 255 characters.

Changes:

• Add ServiceEntrySpanProcessor._on_ending to register the final sw.transaction value with the transaction name pool just before the span becomes immutable.
• Update set_transaction_name() to only set (and truncate) the span attribute; pool registration is deferred to _on_ending.
• Adjust unit/integration tests to reflect the new flow and the 255-character truncation limit.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
solarwinds_apm/trace/serviceentry_processor.py	Defers pool registration to _on_ending and updates default transaction-name setting logic.
solarwinds_apm/api/__init__.py	Stops pool registration in set_transaction_name() and truncates the attribute value to INTL_SWO_TRANSACTION_ATTR_MAX.
tests/unit/test_processors/test_serviceentry_processor.py	Updates unit tests for new set_default_transaction_name signature and default span naming inputs.
tests/unit/test_api.py	Updates set_transaction_name() unit tests to expect direct attribute setting (no pool registration).
tests/integration/test_set_transaction_name.py	Corrects long-name truncation assertions to 255 characters.

Comments suppressed due to low confidence (1)

solarwinds_apm/trace/serviceentry_processor.py:162

• _on_ending() introduces a new critical code path that performs the transaction name pool registration and applies the "other" fallback. There are unit tests for on_start()/on_end(), but none that assert _on_ending() registers exactly once for entry spans, skips non-entry spans, and updates the span attribute based on pool.registered(). Adding targeted unit tests would reduce regression risk for this behavior change.

    def _on_ending(self, span: Span) -> None:
        """
        Finalize transaction name by registering with pool.

        This is called before the span becomes immutable, allowing us to update
        the transaction name attribute with the pool-registered version.