Prepare Metrics for GA

[jamescrosswell]

When Sentry moves Metrics to GA we need to change our APIs so that they don't look intentionally so Experimental.

ExperimentalMetricsOptions could be marked as Obsolete and inherit from a vanilla MetricsOptions class that would replace this (and not be marked obsolete of course). That way code for customers who've been using these experimentally will continue to work without change, but we'll give people a heads up that this will require code changes on their part eventually.

• Preparing Metrics for GA (code changes) #3246
• Updated Metrics Setup docs for the dotnet SDK to prepare for GA sentry-docs#10119

[bitsandfoxes]

Sorry, we won't address this.
Superseded by #3597