feat: Add experimental concurrent processor for logs

[cijothomas]

1. Add a new processor under experimental feature flag.
2. This is to facilitate discussions in OTel spec about adding such a component to the official spec itself.
3. This is NOT useful as-is with any existing exporters in this repo (like OTLP Log Exporter
4. This is an initial version. Next, I'll introduce ways to ensure that only those exporters that can support concurrent exporting will be paired with ConcurrentExporter. This is one possible way spec could evolve. Alternatively, existing processors can be modified to export concurrently if the exporter advertises that they are okay with that!. BatchLogProcessor also need to evolve to support concurrent exporting in the future.
5. Also modified Stress test to use the new experimental processor to compare the perf numbers with Concurrent vs Existing SimpleProcessor.
6. SimpleProcessor did not have a public new method and it did not implement event_enabled. This addresses that also, as it was required to run stress test comparison. Can do this in a separate PR it helps review easier.

Spec discussion:
open-telemetry/opentelemetry-specification#4231

[codecov]

Codecov Report

Attention: Patch coverage is 25.64103% with 29 lines in your changes missing coverage. Please review.

Project coverage is 79.6%. Comparing base (52cd0e9) to head (fe5f455).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...telemetry-sdk/src/logs/concurrent_log_processor.rs	0.0%	28 Missing ⚠️
opentelemetry-sdk/src/logs/simple_log_processor.rs	90.9%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #2780     +/-   ##
=======================================
- Coverage   79.7%   79.6%   -0.1%     
=======================================
  Files        123     124      +1     
  Lines      23136   23174     +38     
=======================================
+ Hits       18448   18456      +8     
- Misses      4688    4718     +30     

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[lalitb]

LGTM.

[ThomsonTan]

Wondering why true is returned to enable events if the exporter cannot be locked?

[cijothomas]

Good point! Not sure what is the right behavior here.
Since true is the default, I opted for true. It can be argued that if the lock got poisoned, no logs can be exported anyway, so returning false also is valid.

Good to revisit this. I'll probably revisit the need of Mutex lock in SimpleLogProcessor, and use atomics to achieve the goal is ensuring one export() is active at a time. With that, we won't face the lock poison issue.