Add Prometheus metrics for artifact uploads

[catkins]

Motivation

• Provide Prometheus metrics for artifact uploads similar to existing log upload metrics so uploads and failures can be monitored.
• Capture both per-artifact counts and uploaded byte totals and ensure failures are reported once per artifact.

Description

• Added internal/artifact/metrics.go which defines Prometheus counters: buildkite_agent_artifacts_uploaded_total, buildkite_agent_artifacts_bytes_uploaded_total, and buildkite_agent_artifacts_uploads_failed_total.
• Wired metrics into artifactUploadWorker.stateUpdater in internal/artifact/uploader.go so an artifact transitioning to "finished" increments uploaded counters and bytes, and the first transition to "error" increments the failure counter.
• Added internal/artifact/metrics_test.go unit tests that exercise successful and failed state transitions and assert metric deltas using Prometheus testutil.
• Updated internal/artifact/BUILD.bazel to include the new test file and add the Prometheus testutil test dependency.

Testing

• Ran go test ./internal/artifact -run TestArtifactMetrics -v which passed all tests.
• Ran go test ./internal/artifact -run TestCollect -v which passed all tests.
• Tests use prometheus/testutil to assert metric values and verified success for both success and failure code paths.

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Codex Task