feat: export OpenTelemetry probe metrics

[stianfro]

Summary

• add explicit, optional OTLP HTTP/protobuf metric export
• export the standard HTTP client duration histogram and HTTPing probe counters
• support standard OTLP environment configuration and sanitized target names
• report background export failures without stopping probes
• prepare the changelog and package metadata for v0.2.0

Verification

• just ci-diff
• local fake OTLP HTTP/protobuf collector test
• manual opt-in, disabled-by-default, and exporter-failure tests