Add embedded private method interfaces in metric API

[MrAlias]

Resolves #3865

This adds an embedded sub-package to otel/metric. It contains interfaces from otel/metric but only with a single unexported method. These interfaces are embedded in all their corresponding interfaces in otel/metric. This means that from the SDK developers perspective they need to ...

1. Embed the appropriate embedded interface if they want their SDK to have a compile time error if an interface from otel/metric has a method added to it.
2. Embed the otel/metric interfaces if they want to have runtime panics
3. Embed another implementation, like the No-Op, if they want to have a default behavior other than panicking

Having the embedded interfaces in their own package removes the distraction for instrumentation authors who have no need to interact with the types in embedded.

[codecov]

Codecov Report

Merging #3916 (ef80f84) into main (5f13db5) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3916     +/-   ##
=======================================
- Coverage   81.7%   81.7%   -0.1%     
=======================================
  Files        171     171             
  Lines      12951   12951             
=======================================
- Hits       10590   10586      -4     
- Misses      2138    2142      +4     
  Partials     223     223             

Impacted Files	Coverage Δ
internal/global/instruments.go	60.8% <ø> (ø)
internal/global/meter.go	93.8% <ø> (ø)
metric/instrument/asyncfloat64.go	100.0% <ø> (ø)
metric/instrument/asyncint64.go	100.0% <ø> (ø)
metric/instrument/syncfloat64.go	100.0% <ø> (ø)
metric/instrument/syncint64.go	100.0% <ø> (ø)
metric/noop/noop.go	100.0% <ø> (ø)
sdk/metric/instrument.go	96.0% <ø> (ø)
sdk/metric/provider.go	100.0% <ø> (ø)
sdk/metric/meter.go	89.5% <100.0%> (ø)
... and 1 more

... and 1 file with indirect coverage changes