rm-hull / dot-block / 19406973011

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feat: Add upstream latency & rename latency metric (#27)

* 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.

* feat: Validate upstream servers in DNS client

The `RoundRobinClient` constructor now validates that at least one
upstream server is configured.

If the upstream list is empty, an error is returned during
initialization, preventing the application from starting with
an invalid configuration. This required updating the signature
of `NewRoundRobinClient` to return an error.

* refactor: Use shared `latencyBuckets` constant

Extracts the Prometheus histogram bucket definition into a shared
package-level variable `latencyBuckets`.

This eliminates duplication and ensures that all relevant DNS
latency metrics use the identical bucket scheme for consistency
and easier comparison.

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