Improve rpc_soak and channel_soak test to cover concurrency in Go

[zbilun]

RELEASE NOTES:

• Improved the rpc_soak and channel_soak tests to support concurrency in Go. This change increases the test coverage of the C2P E2E load test and follows a similar approach used in the Java implementation.

This PR aims to enhance the test coverage of the C2P E2E load test by improving the rpc_soak and channel_soak tests to support concurrency in Go. The updated logic closely follows the approach used in the Java implementation, which has already been merged.

rpc_soak:
The client performs many large_unary RPCs in sequence over the same channel. The test can run in either a concurrent or non-concurrent mode, depending on the number of threads specified (soak_num_threads).

channel_soak:
Similar to rpc_soak, but this time each RPC is performed on a new channel. The channel is created just before each RPC and is destroyed just after.

Note on Concurrent Execution and Channel Creation:
In a concurrent execution setting (i.e., when soak_num_threads > 1), each thread performs a portion of the total soak_iterations and creates and destroys its own channel for each RPC iteration.

Thread-specific logs will include the thread_id, helping to track performance across threads, especially when each thread is managing its own channel lifecycle.

@townba
@apolcyn

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.18%. Comparing base (66ba4b2) to head (3929ecf).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7926      +/-   ##
==========================================
+ Coverage   82.04%   82.18%   +0.13%     
==========================================
  Files         377      379       +2     
  Lines       38180    38260      +80     
==========================================
+ Hits        31326    31445     +119     
+ Misses       5551     5523      -28     
+ Partials     1303     1292      -11     

see 24 files with indirect coverage changes

[townba]

I don't know what you mean by how the channel is handled (reuse or reset). Can you clarify? (Nit: The comment should end with punctuation.)

[zbilun]

It determines whether a new channel will be created or an existing one reused. It replaces the original resetChannel boolean flag.