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refactor: Rename latency metric and add upstream latency

The overall DNS request latency metric is renamed for clarity and
a new metric is introduced to track upstream query latency.

*   Renamed `LatencyHistogram` to `RequestLatency` in `DnsMetrics`.
*   Added `UpstreamLatency` histogram vector to track latency broken
    down by upstream IP address.
*   Updated `RoundRobinClient.Exchange` to return the upstream IP
    address used for the exchange.
*   Updated `DNSDispatcher` to record upstream latency.