doc: migrate away from defunct addMetricReader in examples

[chancancode]

Which problem is this PR solving?

Update READMEs and examples to migrate away from addMetricReader, which was removed in #4419.

Don't think it needs a CHANGELOG entry, since there is already one for #4419 (but it was duplicated for some reason so I fixed it).

Related: open-telemetry/opentelemetry-js-contrib#2646 (comment) (cc @pichlermarc @trentm)

Short description of the changes

Updated to use the constructor option.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Eyeballed it 👁️👄👁️ 🤷🏼‍♂️

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.58%. Comparing base (fc0edd8) to head (3fe51b1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5349   +/-   ##
=======================================
  Coverage   94.58%   94.58%           
=======================================
  Files         318      318           
  Lines        8069     8069           
  Branches     1701     1701           
=======================================
  Hits         7632     7632           
  Misses        437      437           

Files with missing lines	Coverage Δ
...es/shim-opencensus/src/OpenCensusMetricProducer.ts	100.00% <ø> (ø)