Add Datadog support for span-related monitoring functions

[timmc-edx]

[Consider breaking out each method as its own ticket when this is to be picked up.]

Acceptance criteria:

• Complete all of the following for each method:
  • function_trace: feat: Create manual spans for monitoring backends. #435
    • Move implementation into NewRelicBackend.
    • Implement in DatadogBackend.
    • Update function to call all of the telemetry backends.
    • edx_django_utils/monitoring/README.rst updated with latest compatibility info.
    • Inform owners of calling code, especially for backward-incompatible changes in Datadog.
  • set_monitoring_transaction_name
    • Move implementation into NewRelicBackend.
    • Implement in DatadogBackend.
    • Update function to call all of the telemetry backends.
    • edx_django_utils/monitoring/README.rst updated with latest compatibility info.
    • Inform owners of calling code, especially for backward-incompatible changes in Datadog (see notes).
  • ignore_transaction
    • Move implementation into NewRelicBackend.
    • Implement in DatadogBackend.
    • Update function to call all of the telemetry backends.
    • edx_django_utils/monitoring/README.rst updated with latest compatibility info.
    • Inform owners of calling code, especially for backward-incompatible changes in Datadog.
  • Handle get_current_transaction appropriately (see notes).
  • Handle background_task appropriately (see notes).

Implementation notes:

• Optionally, choose new names that are less specific to NR, and deprecate the old names (leaving stubs in their place that emit a warning and call the new function).

Other notes:

• Some repos currently call these directly on newrelic.agent rather than using edx-django-utils's indirection. [APM] Remove direct references to newrelic edx/edx-arch-experiments#621 covers resolving this.
• It appears that get_current_transaction is currently only used internally, despite being exposed, so it can probably be skipped for now.
• Relatedly, background_task appears to be completely unused. (Should this be retired?)
• 2U did not end up using OpenTelemetry, so we can't commit to adding support to OpenTelemetryBackend at the moment. These new methods can be implemented as no-ops in that backend.
• The method set_monitoring_transaction_name is used for XBlocks here, and the resource name will probably change, which may affect monitoring or other XBlock related observability.

[robrap]

Note that function_trace has since been implemented.

[robrap]

Additional Notes:

• What remains is NR transaction-related calls: set_monitoring_transaction_name and ignore_transaction (found in transactions.py)
• The closest thing to a NR transaction in Datadog (and OpenTelemtry) is the Span.
• In Datadog, we probably want set_monitoring_transaction_name to update the resource name only (not operation name). I think the more general name goes to both the resource and operation name.

UPDATE: We need to communicate these changes.

[robrap]

I updated AC and moved this to P3, since these methods are in use. It would be great to confirm and retire what is not in use(see notes).