feat: add optional operation_name to function_trace/create_span

[rahulkanneri-2u]

Description:

Adds an optional operation_name argument to function_trace() and TelemetryBackend.create_span(). This replaces #548, rebased onto 8.0.2 (which implemented OpenTelemetryBackend.create_span()).

• Datadog: uses it as the span operation name and keeps name as the resource.
• OpenTelemetry: records it as an operation_name span attribute, since OTel has no separate operation name.
• New Relic: accepts it and ignores it, because FunctionTrace has no equivalent.

Behavior is unchanged when operation_name is not passed. Also corrects the monitoring README feature matrix, which still listed OpenTelemetry as not supporting span creation since 8.0.2, and adds an operation-name row.

JIRA:

N/A

Dependencies:

Supersedes #548. No other dependencies.

Merge deadline:

None

Installation instructions:

None

Testing instructions:

1. Install test requirements: pip install -r requirements/test.txt
2. Run pytest edx_django_utils/monitoring
3. Expect all tests to pass, including the new test_create_span_with_operation_name, which checks:
   • Datadog: trace('some.operation', resource='some_function_name')
   • OpenTelemetry: start_as_current_span('some_function_name', attributes={'operation_name': 'some.operation'})
   • New Relic: FunctionTrace('some_function_name')
4. If any backend receives different arguments, check failed.

Reviewers:

• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped (8.1.0)
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPi after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

Author concerns:

• Custom TelemetryBackend subclasses must update their create_span signature to accept operation_name=None, otherwise function_trace() will raise a TypeError. This is noted in the changelog and is why the bump is a minor version (8.1.0).
• The OpenTelemetry attribute key (operation_name) is my choice. Happy to rename it (e.g. operation.name) if reviewers prefer a different convention.

[robrap]

Minor possible doc improvement.

[robrap]

Is this accurate? Does this help clarify that this is just a different version of create span?

Suggested change

	* - Operation name for a span (``function_trace(..., operation_name=...)``)
	* - Create span with operation name (``function_trace(..., operation_name=...)``)

[robrap]

If we want to follow-up with a small PR for this, we can. Didn't want you to be blocked on this minor note.