Feature Request: Disable injection of DD tracing headers for some requests

[williamscs]

Are you requesting automatic instrumentation for a framework or library? Please describe.
This is for all HTTP requests.

Is your feature request related to a problem? Please describe.
We have external APIs that we hit which will block our traffic if the request has unexpected headers. There is an option to disable X headers entirely, but this also breaks downstream tracing for internal calls.

Describe the solution you'd like
We would like to either have a blocklist for that partner, or an allowlist for our internal traffic.

Describe alternatives you've considered
Setting DD_TRACE_X_DATADOG_TAGS_MAX_LENGTH to 0 should disable for all HTTP requests.

#3000 (comment)

Additional context
Similar requests in other tickets:

• Can we disable injection of DD tracing headers for some requests? dd-trace-js#3662
• Can we disable injection of DD tracing headers for some requests? dd-trace-java#3685