Prepare for migration to new runtime metrics

[dashpole]

Part of #5655

This is a refactoring to prepare for the implementation of the new runtime metrics. It:

• Adds support for OTEL_GO_X_DEPRECATED_RUNTIME_METRICS, which can be set to true or false to enable/disable the existing runtime metrics. It initially defaults to true while the new metrics are under development.
• Moves the existing runtime metrics implementation to its own internal package, deprecatedruntime, to clearly separate it from the new metrics being added, and to make the eventual removal easier.

This does not change any of the metrics generated, or the public API surface of the runtime metrics package.

[dashpole]

This differs from the "Feature" pattern used elsewhere because I wanted to be able to have a default value of "true". Rather than make the existing pattern more complex to support this, I simplified it to fit this packages' needs.

[dashpole]

This link will not work until after the next release.

[codecov]

Codecov Report

Attention: Patch coverage is 7.34463% with 164 lines in your changes missing coverage. Please review.

Project coverage is 63.9%. Comparing base (a794a70) to head (8c344a3).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5747   +/-   ##
=====================================
  Coverage   63.9%   63.9%           
=====================================
  Files        195     197    +2     
  Lines      12267   12285   +18     
=====================================
+ Hits        7849    7862   +13     
- Misses      4196    4201    +5     
  Partials     222     222           

Files	Coverage Δ
instrumentation/runtime/internal/x/x.go	100.0% <100.0%> (ø)
instrumentation/runtime/runtime.go	0.0% <0.0%> (ø)
...tion/runtime/internal/deprecatedruntime/runtime.go	0.0% <0.0%> (ø)

[MadVikingGod]

This looks good to me.

Are we going to keep the same scope for the deprecated metrics, and will it be the same as the scope of the new metrics?

[dashpole]

I was going to keep the same scope for the deprecated metrics. It doesn't make sense to me to modify the scope if they are going away anyways.

[MrAlias]

Looks like this introduced a broken link. I'll look into submitting a fix.