nginx module produces wrong peer address

Describe your environment

nginx installed as a forward or reverse proxy with the otel instrumentation module installed.

Steps to reproduce

Hit the proxy and inspect the attributes of the generated span.

What is the expected behavior?

The span created by the module when nginx is acting as a proxy should have:

• network.peer.address set to $upstream_addr
• client.address set to $remote_addr

What is the actual behavior?

The span created by the module has the attribute:

• net.peer.ip set to $remote_addr

https://github.com/open-telemetry/opentelemetry-cpp-contrib/blob/22e7fe29f82f2da73b166d7fb7950b56af390a75/instrumentation/nginx/src/otel_ngx_module.cpp#L50

Additional context

• OTel Sem Conv for Networking Attributes: https://opentelemetry.io/docs/specs/semconv/attributes-registry/network/#network-attributes
• $upstream_addr docs: https://nginx.org/en/docs/http/ngx_http_upstream_module.html#var_upstream_addr
• $remote_addr docs: https://nginx.org/en/docs/http/ngx_http_core_module.html#var_remote_addr

Start at instrumentation/nginx/src/otel_ngx_module.cpp around line 50 and compare the current span attributes with the networking semantic conventions. The change is complete when proxied spans use $upstream_addr for network.peer.address and $remote_addr for client.address instead of the current net.peer.ip mapping.