revert: reintroduce external producers

[spoutn1k]

Fixes #2505

Changes

We re-introduce a way to get data from external sources after its removal in #2105.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: spoutn1k / name: Jean-Baptiste Skutnik (3e40ec8)

[codecov]

Codecov Report

Attention: Patch coverage is 31.57895% with 26 lines in your changes missing coverage. Please review.

Project coverage is 80.0%. Comparing base (eac368a) to head (3e40ec8).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/manual_reader.rs	0.0%	19 Missing ⚠️
opentelemetry-sdk/src/metrics/periodic_reader.rs	63.1%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3025     +/-   ##
=======================================
- Coverage   80.0%   80.0%   -0.1%     
=======================================
  Files        126     126             
  Lines      21879   21911     +32     
=======================================
+ Hits       17519   17529     +10     
- Misses      4360    4382     +22     

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