feat(tracing): add otel always_on sampler

[davidspek]

Fixes: istio/istio#60500

This PR adds configuration support for the OpenTelemetry always_on trace sampler. This sampler has been part of the Istio proxy build for 2 years already, but has never been exposed in the configuration.

[istio-policy-bot]

🤔 🐛 You appear to be fixing a bug in Go code, yet your PR doesn't include updates to any test files. Did you forget to add a test?

Courtesy of your friendly test nag.

[istio-policy-bot]

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[istio-testing]

Hi @davidspek. Thanks for your PR.

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[zirain]

what's the different with 100% sampling rate?

[davidspek]

@zirain The difference is that the always_on sampler will change the propagated trace header to indicate that it's sampled. We have issues with an external party passing trace headers that aren't sampled on their side, causing our internal spans to also not be sampled resulting in loss of observability data.

[zirain]

/ok-to-test