feat(sdk): Add Prometheus exposition format export

[lowhung]

Summary

Adds support for exporting metrics in Prometheus text-based exposition format, enabling scraping by Prometheus or compatible monitoring systems.

Changes

• Add prometheus feature flag to buswatch-sdk
• Create prometheus.rs module with:
  • PrometheusConfig builder for configuration
  • PrometheusExporter for storing and rendering metrics
  • format_prometheus() function to convert snapshots to Prometheus format
• Add Output::prometheus() variant
• Integrate HTTP server using hyper to serve metrics endpoint
• Include /health and /healthz endpoints for Kubernetes probes

Metrics Exported

• buswatch_read_count (counter) - Total messages read
• buswatch_write_count (counter) - Total messages written
• buswatch_read_backlog (gauge) - Unread message count
• buswatch_read_pending_seconds (gauge) - Read wait time
• buswatch_write_pending_seconds (gauge) - Write wait time
• buswatch_read_rate_per_second (gauge) - Read throughput
• buswatch_write_rate_per_second (gauge) - Write throughput
• buswatch_snapshot_timestamp_seconds (gauge) - Snapshot timestamp

All metrics include module and topic labels.

Usage

use buswatch_sdk::{Instrumentor, Output};
use buswatch_sdk::prometheus::PrometheusConfig;

#[tokio::main]
async fn main() {
    let config = PrometheusConfig::builder()
        .listen_addr("0.0.0.0:9090")
        .metrics_path("/metrics")
        .build();

    let instrumentor = Instrumentor::builder()
        .output(Output::prometheus(config))
        .build();

    let handle = instrumentor.register("my-service");
    handle.record_read("events", 100);

    instrumentor.start();

    // Metrics available at http://localhost:9090/metrics
}

Closes #10