[AI Gateway] Add New Relic to common OTEL backends

[adjohn]

Summary

Adds New Relic to the list of common OTEL backends on the AI Gateway OpenTelemetry integration page. New Relic ingests OTLP natively over HTTP in both JSON and protobuf with an api-key header, so it works with AI Gateway's OTEL exporter as configured on this page. The bullet links to New Relic's OTLP configuration docs, matching the pattern of the Braintrust, Langfuse, and Datadog entries.

Documentation checklist

• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

✅ No issues found in commit c0e1d46.

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[irvinebroque]

Should this be here too?

https://developers.cloudflare.com/workers/observability/exporting-opentelemetry-data/

[irvinebroque]

@ethulia @nevikashah