Add pagination metrics for known tests fetch

[calvinbayer]

What does this PR do?

Adds three new distribution metrics to track known tests (Early Flake Detection) pagination behavior:

• civisibility.known_tests.pages_fetched - Number of pages fetched during pagination
• civisibility.known_tests.total_fetch_ms - Wall-clock time from first to last page request
• civisibility.known_tests.total_request_ms - Sum of individual per-page request durations

Motivation

Currently, only per-request timing is tracked. These new metrics provide observability into:

• How often pagination occurs (single vs multi-page responses)
• Backend pagination performance (total wall-clock time)
• Network overhead vs local processing time (total_fetch_ms vs total_request_ms)

Implementation Details

• Extended KnownTestsResult struct with three metric fields
• Added metric definitions to TelemetryMetrics
• Updated fetchKnownTests to track timing and emit metrics on success
• Metrics are emitted only on successful completion after all pages are fetched

Testing

• Code compiles and follows Swift patterns
• Requires Swift 6.2 + Xcode for full test execution
• CI will run full test suite

Checklist

• Code complete and follows patterns
• Backward compatible (no breaking changes)
• Follows existing telemetry patterns
• CI tests (requires Swift 6.2 environment)