feat: support SPLUNK_REALM

[seemk]

Description

Add support for realm selection via options or an env var.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• Documentation change or requires a documentation update

How Has This Been Tested?

• Tested manually
• Added automated tests

Checklist:

• Unit tests have been added/updated
• Documentation has been updated
• Change file has been generated (npm run change:new)
• Delete this branch (after the PR is merged)

[codecov-commenter]

Codecov Report

Merging #530 (1965158) into main (70df9b1) will increase coverage by 33.65%.
The diff coverage is 91.93%.

❗ Current head 1965158 differs from pull request most recent head d62e2d5. Consider uploading reports for the commit d62e2d5 to get more accurate results

@@             Coverage Diff             @@
##             main     #530       +/-   ##
===========================================
+ Coverage   54.90%   88.56%   +33.65%     
===========================================
  Files          28       27        -1     
  Lines        2426      918     -1508     
  Branches      919      210      -709     
===========================================
- Hits         1332      813      -519     
+ Misses       1094      105      -989     

Impacted Files	Coverage Δ
src/tracing/options.ts	95.52% <90.56%> (-2.40%)	⬇️
src/metrics/index.ts	92.85% <100.00%> (+0.46%)	⬆️
src/profiling/proto/profile.js

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