Remove MetricProducer

[cijothomas]

Removes MetricProducer, which is an implementation of the MetricProducer from the spec, an optional thing, which allowed metrics from external sources to be sent to OpenTelemetry. This existed for a while, but had no tests or documentation that I can find.
Removing this to keep the scope for 1.0 stable minimized, and to make it slightly easier to do further refactoring.

This is the beginning of a major set of cleanups from Metric collect/reader side of things.

Part of #1719

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.3%. Comparing base (5873cae) to head (0cd926b).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/manual_reader.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2105   +/-   ##
=====================================
  Coverage   78.2%   78.3%           
=====================================
  Files        121     121           
  Lines      20849   20819   -30     
=====================================
- Hits       16315   16310    -5     
+ Misses      4534    4509   -25     

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