Add OTEL_EXPORTER_OTLP_LOGS_* environment variables

[alanwest]

Fixes #2781

Question: OTEL_EXPORTER_OTLP_LOGS_* or OTEL_EXPORTER_OTLP_LOGRECORDS_*?

[reyang]

I would vote for _LOGS_ instead of _LOGRECORDS_ considering folks who configure these environment variables are not necessarily developers who need to handle the data type / programming model. (e.g. we also use _TRACES_ instead of _SPANS_)

[reyang]

Couple things to consider:

1. Do we want to mark these as Experimental for now, and turn it to Stable later? (I think it's okay to mark these Stable)
2. Changelog?
3. Update the feature matrix?

[alanwest]

1. Do we want to mark these as Experimental for now, and turn it to Stable later? (I think it's okay to mark these Stable)

My thought was that stable should be fine as well.

2. Changelog?

Updated.

3. Update the feature matrix?

The feature matrix does not currently callout the signal-specific OTLP exporter env vars

opentelemetry-specification/spec-compliance-matrix.md

Line 275 in 5f772ee

|OTEL_EXPORTER_OTLP_* | + | + | | + | + | + | - | + | + | + | - |

I guess we could add them though?