Remove field-map copying from default request access logs

[perfloop-agent]

Avoid building and copying field maps for default text access logs when no supplemental fields are present. Keep the existing Logrus emission path and field-based behavior for JSON, custom formatters, and supplemental fields.

Measurements

Ten paired runs per workload with BenchmarkAccessLogWithOutput:

Workload	Before B/op	After B/op	Change
Inline 200, 10-byte body	18,115	15,410	-14.9%
Same with enableAccessLog(1, 200, 3)	19,779	17,075	-13.7%
Inline 418, 28-byte body	18,091	15,386	-15.0%
setQuery(lang, pt), local 12-byte backend	25,474	22,753	-10.7%
Redirect 308	16,691	13,970	-16.3%
Same after modPath	18,084	15,364	-15.0%

The benchmark measures complete Proxy.ServeHTTP calls with synchronous access-log writes to a regular file. Setup is outside the timed loop; the query backend is warmed. B/op includes response-recorder and loopback transport/backend allocations. Production traffic weights remain unmeasured. Each row requires at least 10% lower B/op with no ns/op or allocs/op regression.

Validation

Formatting, lint, logging/access-log tests, focused proxy tests, and logging/concurrent-access-log race checks passed.

TestAccessLogOnFailedRequest returns 407 instead of 502 and leaves two persistent-connection goroutines on both the baseline and this change; the cause is unresolved. Full repository CI remains pending. make shortcheck was not run because the Redis Testcontainers tests require Docker, which is unavailable in the sandbox.

Fixes #4325

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Generated by Perfloop. Human sponsor: Tomás Senart.

[szuecs]

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[a4180p]

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