Track all read latencies in TransactionMetrics

The client `TransactionMetrics` trace event logs some details about read latencies, but these latencies are only for single-key get requests. It would be nice if we had details about all read requests (e.g. getRange, getKey).

Start with the TransactionMetrics trace event and inspect how latency details are currently recorded for single-key get requests. Trace the handling of getRange and getKey reads, then verify that all read requests produce the expected latency details in the transaction metrics output.

Written by the indexing model from the issue text.