fix: skip req metadata serialization in general case

[ferhatelmas]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

reqMetadata is empty most of the time so assign string directly to skip json stringify/pino serializer overhead.

What is the new behavior?

Update type to string and initialize with empty json shape. No log changes but faster to log.

Additional context

It's serialized once when tracing is enabled, it was done twice before.

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[coveralls]

Coverage Report for CI Build 28433157058

Coverage increased (+0.005%) to 78.811%

Details

• Coverage increased (+0.005%) from the base build.
• Patch coverage: 1 uncovered change across 1 file (4 of 5 lines covered, 80.0%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
src/http/plugins/log-request.ts	5	4	80.0%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	12201
Covered Lines:	10057
Line Coverage:	82.43%
Relevant Branches:	7026
Covered Branches:	5096
Branch Coverage:	72.53%
Branches in Coverage %:	Yes
Coverage Strength:	423.01 hits per line

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💛 - Coveralls