feat: Add metrics/instrumentation

[tomverelst]

Problem

Currently there's no observability beyond logging. Operators have limited visibility into performance and progress during large migrations.

Suggestion

Add metrics using the metrics crate or OpenTelemetry:

Counters

• kbridge_offsets_processed_total - Number of offsets processed
• kbridge_partitions_scanned_total - Number of partitions scanned
• kbridge_binary_search_iterations_total - Binary search iterations performed

Histograms

• kbridge_operation_duration_seconds - Time per operation (fetch/calculate/apply)
• kbridge_partition_scan_duration_seconds - Time to scan a partition

Gauges

• kbridge_pending_offsets - Offsets remaining to process

Example

use metrics::{counter, histogram};

counter!("kbridge_offsets_processed_total").increment(1);
histogram!("kbridge_operation_duration_seconds").record(duration.as_secs_f64());

Impact

Enables monitoring, alerting, and performance analysis for production use.