Deprecation notice for B3 propagation configuration

[marcotc]

This PR renames the configuration options for the distributed tracing propagation styles B3 and B3 single header to align with OpenTelemetry.

The existing names will still be supported, but a deprecation warning will be issued.

There are no changes the propagation styles themselves, only configuration name changes.

A change to the environment variables used to configure propagation styles was also done, as to standardize them across other Datadog languages.

Changes

• B3 becomes b3multi, as is represent B3 over multiple headers. B3 remains a valid, but deprecated alias.
• B3 single header becomes b3. This does not conflict with B3 as this configuration is case sensitive. B3 single header remains a valid, but deprecated alias.
• DD_PROPAGATION_STYLE_INJECT becomes DD_TRACE_PROPAGATION_STYLE_INJECT. DD_PROPAGATION_STYLE_INJECT remains as a valid, but deprecated alias.
• DD_PROPAGATION_STYLE_EXTRACT becomes DD_TRACE_PROPAGATION_STYLE_EXTRACT. DD_PROPAGATION_STYLE_EXTRACT remains as a valid, but deprecated alias.

One more thing

I introduced a formal deprecation logger:

Datadog::Core.log_deprecation { "This is not good anymore. Do this better thing instead." }

By default, it logs to Datadog.logger.warn.

I also created a simple way to assert it:

include_examples 'records deprecated action', 'This is not good'