[metrics] Add scanner fetch and remote download metrics for client observability

[charlesdong1991]

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Description

Metrics framework is implemented, so we can add more metrics to enhance observability.

Java's ScannerMetricGroup exposes per-table metrics so as to get visibility into rpc latency, throughput, or remote segment download healthiness.

This issue aims to adding the fetch + remote download metrics.

Willingness to contribute

• I'm willing to submit a PR!