Correct scheduled slots documentation and missing open telemetry span

[mattogburke]

The new pool.scheduled_slots metrics added in #38899 has incorrect documentation, calling the field pool.scheduled_tasks.

Additionally this field isn't exposed in the open telemetry span data unlike all the other related pool.{state}_slots metrics. I suspect this was missed in #40802

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[boring-cyborg]

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[potiuk]

cc: @howardyoo @ferruzzi ?

[howardyoo]

cc: @howardyoo @ferruzzi ?

The change looks good to me. Thanks for adding these missing attributes to the span :-)

[ferruzzi]

LGTM. The citations to the definitions in the code are a very nice touch, thanks for those.

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.

[jedcunningham]

Thanks @mattogburke! Congrats on your first commit 🎉