Configuration for instrumentations through environment variables

[svrnm]

This issue is blocking #2461. Based on the SIG meeting yesterday the overall consensus was that there is a need for a configuration specification (see #1773). Assuming that this will take some time, the question is what to do with existing & future instrumentation configurations? The example with the HTTP headers in #2461 shows that it would be valuable to have a standard here that is consistent across languages.

Copying the detail questions from @tigrannajaryan's comment:

1. Should instrumentations be configurable via env variables?
2. Is the same configuration settable from the code?
3. Is naming of instrumentations' env variables dictated by the spec?
4. Where in the spec it should live?
5. What is the variable naming scheme? Do we choose a particular prefix like this PR suggests?
6. How to name variables which are specific for just one instrumentation and are not generic and applicable to many instrumentations like "OTEL_INSTRUMENTATION_HTTP*" is? Should the variable names be prefixed by the instrumentation library name?