fix(2775): send metrics to posthog

[beelchester]

fixes: #2775
/claim #2775

---

Steps to test:

1. Add posthog project api key to env POSTHOG_API_SECRET.
2. For local testing I had to hardcode is_tracking to true.
3. Run cargo test on tailcall-tracker or just use build the project and run any argument.
4. Check the activity section in posthog project events should appear.

[codecov]

Codecov Report

Attention: Patch coverage is 19.63190% with 131 lines in your changes missing coverage. Please review.

Project coverage is 86.95%. Comparing base (d02b6e6) to head (84af049).

Files with missing lines	Patch %	Lines
tailcall-tracker/src/collect/collectors.rs	13.20%	46 Missing ⚠️
tailcall-tracker/src/collect/posthog.rs	9.09%	30 Missing ⚠️
tailcall-tracker/src/collect/ga.rs	20.58%	27 Missing ⚠️
tailcall-tracker/src/tracker.rs	57.69%	11 Missing ⚠️
src/cli/command.rs	0.00%	7 Missing ⚠️
src/cli/tc/run.rs	0.00%	4 Missing ⚠️
tailcall-tracker/src/collect/ga_event.rs	0.00%	4 Missing ⚠️
tailcall-tracker/src/error.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2776      +/-   ##
==========================================
- Coverage   87.13%   86.95%   -0.18%     
==========================================
  Files         254      257       +3     
  Lines       25144    25250     +106     
==========================================
+ Hits        21910    21957      +47     
- Misses       3234     3293      +59     

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[tusharmath]

• Added comments
• CI is failing which needs fixing
• Add steps to test the feature

Moving to draft to reduce noise and improve CI efficiency. Once you are ready just mark it as "ready to review". Feel free to give a shoutout on the #contributors channel on Discord if you want immediate attention.

[github-actions]

Action required: PR inactive for 5 days.
Status update or closure in 10 days.