accesslog: remove ALS gRPC initialMetadata

[zirain]

fixes: #3737

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.96%. Comparing base (b799c08) to head (e26dee0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3751      +/-   ##
==========================================
- Coverage   68.99%   68.96%   -0.03%     
==========================================
  Files         176      176              
  Lines       21743    21730      -13     
==========================================
- Hits        15002    14987      -15     
- Misses       5661     5662       +1     
- Partials     1080     1081       +1     

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[zhaohuabing]

@zirain Could you please explain in the comment why this fixed #3737 ?

[zirain]

@zirain Could you please explain in the comment why this fixed #3737 ?

        // Header value.
	//
	// The same :ref:`format specifier <config_access_log_format>` as used for
	// :ref:`HTTP access logging <config_access_log>` applies here, however
	// unknown header values are replaced with the empty string instead of “-“.
	// Header value is encoded as string. This does not work for non-utf8 characters.
	// Only one of “value“ or “raw_value“ can be set.

Header valus support access log format, but didn't support extension formatter(e.g. METADATA, CEL etc.)
InitialMetadata is designed to send something for auth(e.g. got token from request header)
ALS doesn't need it in current stage.