InternalMetrics- conditionally start dispatch thread if there are any readers

[cijothomas]

Alternate to #1552
Since perf tests don't create service/telemetry, this will also unblock perf tests.

Simple optimization- avoid the thread creation if there are no readers.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.76%. Comparing base (01fa772) to head (9aa8318).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1555   +/-   ##
=======================================
  Coverage   83.76%   83.76%           
=======================================
  Files         433      433           
  Lines      122633   122639    +6     
=======================================
+ Hits       102719   102732   +13     
+ Misses      19380    19373    -7     
  Partials      534      534           

Components	Coverage Δ
otap-dataflow	85.06% <0.00%> (+0.01%)	⬆️
query_abstraction	80.61% <ø> (ø)
query_engine	90.31% <ø> (ø)
syslog_cef_receivers	∅ <ø> (∅)
otel-arrow-go	53.50% <ø> (ø)

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