Disable tracing via context

[mwear]

We should have a mechanism to disable tracing via a flag on the context. Other SIGs are considering similar approaches. See: open-telemetry/opentelemetry-js#1040.

[johnnyshields]

This is probably needed for the Elasticsearch tracer. ES gem uses Faraday or Net::HTTP under the hood to talk to ES so we should probably disable that one, otherwise we get rather meaningless duplicate traces.

[github-actions]

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