perf(telemetry): serialize batches on shared thread pool

[jpnurmi]

Let the telemetry module own a shared thread pool for telemetry batchers, and use it for log and metric batch serialization.

Before: on the left, the batcher thread is unable to keep up with serializing incoming logs before handing them off to the transport thread. Serialization takes too much time, queues fill up, logs overflow, and get discarded. The batcher thread continues working for a good while after logs stop coming in.

After: on the right, the batcher thread only acts as a gateway for incoming logs. It has no problem keeping up with the flood because they are only quickly organized into batches and immediately handed over to the telemetry pool, which then distributes the heavy serialization work to its pooled threads, resulting in no discarded logs.

before-vs-after.ftrace.zip

A partial, batcher-side fix to:

• Logs: optimize performance #1862

[codecov]

Codecov Report

❌ Patch coverage is 87.10801% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.54%. Comparing base (38fb099) to head (2f2e296).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1946      +/-   ##
==========================================
+ Coverage   74.31%   74.54%   +0.23%     
==========================================
  Files         104      104              
  Lines       25738    25966     +228     
  Branches     4648     4688      +40     
==========================================
+ Hits        19126    19357     +231     
+ Misses       5305     5299       -6     
- Partials     1307     1310       +3     

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[jpnurmi]

Update: I'm investigating why TSAN jobs started hanging in the CI after the latest rebase and max_pending changes. I haven't been able to reproduce locally... :(