Facilitate no-code instrumentation

https://github.com/couchbase/couchnode/blob/7378b6e1d2681b9b52f547c36eb3df1971f98fbc/lib/cluster.ts#L1080

Since couchbase has decided to instrument the code with OTEL from within the library, that makes it impossible for many consumers to enable OTEL for couchbase across many of their services in a "No-Code" fashion.

To enable otel, the connect configurations may need to be changed across hundreds of services, therefore lengthening the path for customers to enable OTEL, whereas instrumentation libraries allow for a high degree of separation of telemetry intialization & business code.

Therefore please consider 2 points:

• allow tracing config to be configured from env vars (OTEL_CB_TRACING: true or infer that OTEL is enabled from OTEL supported env vars)
• default to fetch the tracer from a registered tracer provider through OTEL trace api.

Those changes would allow customers to benefit from OTEL without any changes to their code.

Start at lib/cluster.ts around line 1080 and review how tracing is configured during connection setup. Check the linked OpenTelemetry SDK environment-variable specification and the OpenTelemetry trace API. Done means tracing can be enabled through supported environment variables and the tracer is obtained from a registered provider without changing consumer connection configuration.