Add `OTEL_DOTNET_AUTO_EXPERIMENTAL` env var

[pellared]

Based on #2416 (comment)

We can add OTEL_DOTNET_AUTO_EXPERIMENTAL that could control if experimental elements should be enabled or not by default.

Currently, all experimental features/instrumentations are enabled by default. Therefore, the default value of OTEL_DOTNET_AUTO_EXPERIMENTAL should be true in order not to introduce a "breaking configuration change".

[rajkumar-rangaraj]

SIG - Proposal is accepted.

[pellared]

According to the spec SIG meeting notes from last week it looks like CNCF and some users do not like that (stable) automatic instrumentation produces unstable instrumentation. I think addressing this issue could be a way of mitigating CNCF an user concerns.

CC @open-telemetry/technical-committee @open-telemetry/governance-committee

[pellared]

Related:

• add blog post announcing proposed changes/oteps for stability work opentelemetry.io#8208