fix: ensure run-context for mysql span does not spill into user code

[trentm]

This fixes the 'mysql' instrumentation to not have the mysql span
context be active in user code. This ensures that user code cannot
create a child span of the mysql span, which would (a) be misleading
and (b) cause problems for coming exit span and compressed span work.

Refs: #2430

Checklist

• Implement code
• Add tests
• Add CHANGELOG.asciidoc entry
• Commit message follows commit guidelines

[apmmachine]

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• Start Time: 2021-11-23T00:56:53.198+0000

• Duration: 21 min 45 sec

• Commit: 9b81f5c

Test stats 🧪

Test	Results
Failed	0
Passed	22
Skipped	0
Total	22

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[trentm]

This passed a full TAV=mysql ./node_modules/.bin/tav run using node v12.22.6 on my machine.

[astorm]

👍 does what is says, approving.

[astorm]

+1 for bailing out early if the span didn't start -- there doesn't appear to be any significant code path that follows in the function for an undefined span.