feat: Add preconfigured support for Smart Agent

[jtmal-signalfx]

No description provided.

[tsloughter-splunk]

The issue where exporters set with the environment variable can't be external also exists for the propagators. The argument was that it is only the environment variable and you are suppose to set custom propagators through the configuration setup code. Is that possible for exporters as well? If not that may be what we want to contribute back.

Though I recall not using the environment variables causes weirdness and that is why I had to do a lot through them in the existing setup code... I'll have to look into this more Monday and can ping some people. Unless you already know if this is just not possible at this time but technically could be.

[jtmal-signalfx]

The issue where exporters set with the environment variable can't be external also exists for the propagators. The argument was that it is only the environment variable and you are suppose to set custom propagators through the configuration setup code. Is that possible for exporters as well? If not that may be what we want to contribute back.

Sent open-telemetry/opentelemetry-ruby#1365

[theletterf]

There's still a docs suggestion to merge

Changes included manually