Add activity worker thread count metrics

[Shaddoll]

What?

Add activity worker thread count metrics to track the number of concurrent activities in a worker.

[coveralls]

Pull Request Test Coverage Report for Build 637

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 71.833%

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Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/uber/cadence/internal/sync/WorkflowThreadContext.java	1	82.46%

Totals
Change from base Build 632:	0.02%
Covered Lines:	10670
Relevant Lines:	14854

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💛 - Coveralls