Standalone ASM priority sampler and tag propagation

[iunanua]

What does this PR do?

Continuation of https://github.com/DataDog/dd-trace-js/pull/4291

Defines two new channels in opentracing/propagation/text_map.js in order to alter trace propagation:

• dd-trace:span:inject which publishes the trace span context and the carrier that is going to be injected in a downstream req
• dd-trace:span:extract which publishes the trace span context and the carrier when a span is going to be extracted from an upstream req.

When standalone ASM is enabled it:

• replaces default PrioritySampler with StandaloneAsmPrioritySampler via DatadogTracer constructor.
  • StandaloneAsmPrioritySampler searches for _dd.p.appsec in the tags in order to keep a trace, sets a 1 req/min rate limiter and disables user rules.
• when injecting a trace and if the trace doesn't contain _dd.p.appsec tag, removes x-datadog- and datadog info from tracestate
• when extracting a trace if the trace doesn't contain _dd.p.appsec tag, resets the sampling priority.
• disables span stats and includes 'Datadog-Client-Computed-Stats': 'yes' header when exporting traces to the agent to pretend stats computation is made by the tracer

ST PR to enable Standalone ASM tests

Motivation

Support Standalone ASM billing

Plugin Checklist

• [ ] Unit tests.
• [ ] TypeScript definitions.
• [ ] TypeScript tests.
• [ ] API documentation.
• [ ] CircleCI jobs/workflows.
• [ ] Plugin is exported.

Additional Notes