There are circumstances for instrumented actions where:

1. The action is frequent and of low interest: a healthcheck, polling a message queue, etc.
2. An OpenTracing exporter uses libraries that themselves may be instrumented (risking infinite tracing).
3. If the current layer (e.g. RPC) happens to offer sufficiently detailed tracing, lower HTTP/DNS/TCP/UDP layers do not need to be traced when invoked with this library. This need is heighten by the use of server/client spans, which AFAIK needs to remove spans in between them. (SpanKind with layers #526).

(1) has come up in #173

(2) has been a problem in opentelemetry-js (open-telemetry/opentelemetry-js#332) HTTP-based exporters, since the Node.js stdlib is instrumented globally. The solution in that has was adding a special HTTP header x-opentelemetry-outgoing-request headers, that the http instrumentation ignores. In essence it uses HTTP headers as a poor-man's context API. (This may not scale to other APIs.)

It would be nice to cut off all automatic tracing "below this current scope" by setting a context key. The default tracer implementation would create no-op spans if the context disables spans.

EDIT: Case (1) may call for sampling, rather than full disabling.