Add ClientCount to ClusterControllerMetrics

[sfc-gh-abeamon]

This adds a client count metric to ClusterControllerMetrics so that it can be viewed in logs in addition to status. This is implemented by iterating through the client map every minute and counting the clients. This is roughly the same procedure that the cluster controller does when it receives a status request.

Passed 10K correctness. Checked with a manual deployment that the client count increases and decreases correctly.

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[foundationdb-ci]

AWS CodeBuild CI Report for macOS Catalina 10.15

• CodeBuild project: foundationdb-pull-request-build-macos
• Commit ID: 15f43ae
• Result: SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)

[foundationdb-ci]

AWS CodeBuild CI Report for Linux CentOS 7

• CodeBuild project: foundationdb-pull-request-build
• Commit ID: 15f43ae
• Result: SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)