feat(cubestore): add OpenTelemetry metrics exporter

[deedubs]

Check List

• Tests have been run in packages where changes have been made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Description of Changes Made (if issue reference is not provided)

Adds an OpenTelemetry (OTLP) metrics exporter to the cubestore process as an
alternative to the existing StatsD/DogStatsD UDP sink.

• New backend, same reporting API. Introduces a MetricSink trait so the
  process-wide metrics sink can be either the StatsD UDP sink or the new
  OpenTelemetry sink. All existing app_metrics call sites (counters, gauges,
  histograms) are unchanged.
• util/metrics/otel.rs. New OtelSink that lazily creates and caches
  OpenTelemetry instruments by metric name, maps DogStatsD-style name:value
  tags to OTLP attributes, and pushes over OTLP/HTTP via a periodic exporter.
• Selection & config. Set CUBESTORE_METRICS_FORMAT=otel to enable it. The
  exporter is initialized inside the Tokio runtime (it is push-based and needs
  an async runtime). Endpoint/interval are configurable via
  CUBESTORE_METRICS_OTEL_ENDPOINT and CUBESTORE_METRICS_OTEL_INTERVAL_SECS,
  falling back to the standard OTEL_EXPORTER_OTLP_* environment variables.
• Docs. Documents the new otel format value and env vars in the
  environment-variables reference.
• Fork process. Adds a "Syncing with Upstream" section to the root
  CLAUDE.md describing how our organization-specific changes are maintained on
  the long-running universe branch (rebased on upstream cube-js/cube master,
  synced on demand via a chore: Sync release PR).

Verification

• cargo check -p cubestore --bin cubestored compiles cleanly.
• rustfmt run on all changed Rust files (clean).
• Added a parse_tag unit test. The full cargo test suite was not run in this
  environment (network-restricted dependency fetch); reviewers should run the
  cubestore test suite in CI.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KNKdpuR2ibDAX8eWJwUcLm

---

Generated by Claude Code