Fix error condition in v2 proxy support

[cdupuis]

Proposed changes

This PR fixes an error condition that seems to have been introduced in cbaa068.

Tests

Following the tests from #979, I see the following error which is consistent with the error condition:

./nginx-prometheus-exporter \
  --nginx.proxy-protocol \
  --nginx.scrape-uri="http://127.0.0.1:18080/stub_status" \
  --web.listen-address=":9116" \
  --web.telemetry-path="/metrics"
time=2025-10-08T08:50:46.977+02:00 level=INFO source=exporter.go:126 msg=nginx-prometheus-exporter version="(version=, branch=, revision=064e74611406c672b3c2403bcdd55cd24d47876d-modified)"
time=2025-10-08T08:50:46.978+02:00 level=INFO source=exporter.go:127 msg="build context" build_context="(go=go1.25.0, platform=darwin/arm64, user=, date=, tags=unknown)"
time=2025-10-08T08:50:46.979+02:00 level=INFO source=tls_config.go:346 msg="Listening on" address=[::]:9116
time=2025-10-08T08:50:46.979+02:00 level=INFO source=tls_config.go:349 msg="TLS is disabled." http2=false address=[::]:9116
time=2025-10-08T08:51:17.609+02:00 level=ERROR source=nginx.go:57 msg="error getting stats" error="failed to get http://127.0.0.1:18080/stub_status: Get \"http://127.0.0.1:18080/stub_status\": round trip failed: writing proxyproto header via tcp to 127.0.0.1:18080: %!w(<nil>)"

With the change in this PR I can successfully run through the series of tests outlined in #979.

Checklist

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• I have read the CONTRIBUTING guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[nginx-bot]

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[github-actions]

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[cdupuis]

I have hereby read the F5 CLA and agree to its terms