AKCORE-184: Share coordinator and coordinator runtime metrics. [4/N]

[smjn]

• Added write-total and write-rate metrics to track number of write state RPCs.
• Added write-latency-avg and write-latency-total to track time taken for write state RPC calls.
• Added share state group num-partition, partition-load-time, idle-thread metrics (via ShareCoordinatorRuntimeMetrics).
• Added tests wherever applicable.

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