Adopt the instrumentation framework for dsx-exchange-consumer counters

[chet]

The DSX exchange consumer's message counters are hand-rolled instruments, several sitting right beside a matching log line. Folding them into the instrumentation framework brings the consumer inline with the rest of the fleet, with no change to any exposed metric.

What this involves

• crates/dsx-exchange-consumer/src/metrics.rs plus the call sites in mqtt_consumer.rs / health_updater.rs: convert the four label-free counters (messages_received_total, messages_processed_total, messages_dropped_total, dedup_skipped_total) to #[derive(Event)] events. The binary already installs the global meter, so framework emits export as-is.
• Where a counter sits beside a log line -- the queue-full warn! and the dedup trace! -- the event owns that line at its historical level.
• Leave alerts_detected_total out of this PR: its point_type label is a caller-supplied string that needs a hand-written bounded mapping, which deserves its own change.
• Preserve every exposed metric exactly -- names (already _total-suffixed), types, and the binary's own /metrics output byte-identical before and after; confirm no golden fixture moves.

Part of #3169.