NGINX: Allow overriding `opentelemetry_trace_id` from within the config

Is your feature request related to a problem?
We use Google Load Balancers which add a vendor specific header (X-Cloud-Trace-Context) to our requests. It seems compatible with W3C format, but OTEL library does not detect it as it's not supported and creates a new span which gets passed down the line (together with Google header), resulting in two separate traces which are not connected.

To solve it, we tried is parsing the X-Cloud-Trace-Context header within NGINX and extracting trace ID and using that to construct a new header which is passed down the line:

http {
      # omitted: some parsing code to $trace_id from X-Cloud-Trace-Context header
      server {
          opentelemetry_propagate;
          opentelemetry_trust_incoming_spans on; 
      location / { 
          proxy_pass http://some_endpoint;

          # Construct a custom header using Google's trace ID and OTEL's generated Span ID
          set $custom_traceparent "00-${trace_id}-${opentelemetry_span_id}-01";

          # Set header
          proxy_set_header traceparent $custom_traceparent;  
        }
    } 
}

This solution passes a valid trace down the line, however it also results in two traces due to OTEL library generating it's own opentelemetry_trace_id which it then sends to the OLTP exporter host. It's not entirely clear to me at what point in the NGINX request flow OTEL generates its trace ID and I'm wondering if there is a way to override it. This can be visualised as follows:

graph TD
    A(Request) -->B(Google Load Balancer)
    B --> |Google adds: X-Cloud-Trace-Context|C(NGiNX)
    C -->|Custom traceparent - Trace A| D(Application)
    C -->|Default/Internal traceparent - Trace B| E(OLTP exporter host)

Describe the solution you'd like

• Approach 1: Ideal solution would be to find out if there's a way to generate and set headers within NGINX before the OtelGetTraceId function gets called (code).
• Approach 2: Alternatively, since NGINX does not seem to support overriding already set variables, perhaps the library functionality could be extended to allow overriding this value from within the code.

Thanks and I hope you can provide some additional context how to best solve it!

Start with instrumentation/nginx/src/otel_ngx_module.cpp at the linked OtelGetTraceId function, then trace when the NGINX module generates and exports its trace ID relative to configuration processing. Done means the configuration can use the Google-derived trace ID consistently for propagation and OLTP export without creating a second trace.