fix(exporter-jaeger): add env variable for agent port

[blumamir]

Which problem is this PR solving?

Exporter Jaeger did not support the OTEL_EXPORTER_JAEGER_AGENT_PORT environment variable from spec.

Short description of the changes

Added this missing environment variable similar to the existing variable OTEL_EXPORTER_JAEGER_AGENT_HOST.

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?

Updated the relevant unit tests with the same pattern as the existing tests for agent host.

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated (no need)

[codecov]

Codecov Report

Merging #2754 (913e111) into main (0dc4c3d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2754   +/-   ##
=======================================
  Coverage   93.27%   93.27%           
=======================================
  Files         159      159           
  Lines        5444     5445    +1     
  Branches     1142     1143    +1     
=======================================
+ Hits         5078     5079    +1     
  Misses        366      366           

Impacted Files	Coverage Δ
...ckages/opentelemetry-core/src/utils/environment.ts	96.00% <ø> (ø)
...ckages/opentelemetry-exporter-jaeger/src/jaeger.ts	94.66% <100.00%> (+0.07%)	⬆️

[blumamir]

node-tests-stable (16) fail but it does not seem to be related to my changes.
tests pass on my local setup with node 16.
is it a known issue?
It was a temporary glitch