Always track long running traces when feature is enabled, regardless of agent support

[deejgregor]

What Does This Do

Currently, the tracer will silently drop long running traces--even if the feature is enabled--when it can't connect to an agent or when the agent doesn't support the long running traces feature.

Motivation

Previously the long running traces buffer would always be empty, even though the feature was enabled with dd.trace.experimental.long-running.enabled=true. This led to a good amount of confusion when I was initially developing a feature to dump long running traces without a local Datadog Agent running.

Additional Notes

This was originally part of #9874, which is being broken out into a few individual PRs.

Commit notes:

• Warn when long running trace feature is enabled but not supported --
• Always track long running traces when feature is enabled -- This could be dropped, but if so, I'd highly suggest keeping the previous commit that adds the warning message. If kept, this should probably be squashed with the previous commit. Note: if features.supportsLongRunning() is false, the traces are kept in the TRACKED state, compared to the NOT_TRACKED state and then immediately removed with cleanSlot, previously.
• Add long-running traces metric for drops due to sampling priority -- This could be dropped. I'm not sure if this is an important metric to track.

Assuming #10309 is merged, the only remaining case where a metric isn't tracked is when an entry is removed from the tracker is when the trace list is empty:

      if (trace.empty()) {
        trace.compareAndSetLongRunningState(WRITE_RUNNING_SPANS, NOT_TRACKED);
        cleanSlot(i);
        continue;
      }

If this case is desired to have a metric, let me know, and I'll add it (long-running.completed??). That would cover every single case where a trace is removed from the tracker.

Contributor Checklist

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Jira ticket: [PROJ-IDENT]

[deejgregor]

Marking this as a draft to prevent merging for now, as I remember seeing the warning generated in one case when the tracer could talk to a modern agent, so I want to look more into that.

Otherwise, feel free to review.