feat(ext/telemetry): support OTEL_TRACES_SAMPLER (#34764)
This implements the `OTEL_TRACES_SAMPLER` and `OTEL_TRACES_SAMPLER_ARG`
environment variables from the OpenTelemetry SDK configuration spec, so
the head-based trace sampler can be configured (previously every span
was always recorded and exported).
Supported `OTEL_TRACES_SAMPLER` values (per spec):
- `always_on`
- `always_off`
- `traceidratio` (probability taken from `OTEL_TRACES_SAMPLER_ARG`,
default `1.0`)
- `parentbased_always_on`
- `parentbased_always_off`
- `parentbased_traceidratio`
