Replace Sampler with ExtendedSampler

[axw]

Since we're making breaking changes, replace the Sampler interface with ExtendedSampler.

[apmmachine]

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Build stats

• Reason: null

• Start Time: 2022-02-03T05:28:50.203+0000

• Duration: 51 min 17 sec

• Commit: ed05fc4

Test stats 🧪

Test	Results
Failed	0
Passed	7206
Skipped	174
Total	7380

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[simitt]

I don't believe we expect this interface to be used externally, so I labeled with test-plan-skip; let me know if you disagree.