[FEATURE]: datadog_service_name NGINX variables support

[shugg]

Describe the goal of the feature

It would be great if the datadog_service_name directive supported NGINX variables. With nginx-datadog on Ingress-NGINX, this would enable dynamically assigning a service name to each ingress object.

controller:
  config:
    main-snippet: "load_module /modules_mount/ngx_http_datadog_module.so;"
    http-snippet: |
      server {
        datadog_sample_rate 0.1;
      }

    location-snippet: |
      datadog_service_name "$ingress_name-ingress";

Is your feature request related to a problem?

No response

Describe alternatives you've considered

No response

Additional context

We are looking to transition from our current setup, which uses the OpenTelemetry module (and previously OpenTracing) bundled in Ingress-NGINX, to the nginx-datadog module for improved support. Here is our current configuration:

OpenTelemetry

location-snippet: |
  opentracing_tag service.name "$ingress_name-ingress";

OpenTracing

location-snippet: |
  opentelemetry_attribute "service.name" "$ingress_name-ingress";

[dmehala]

Hey @shugg

I actually considered a similar use case - automatically resolving $service_name/$ingress_name-ingress for an ingress-nginx runtime. However, during my testing, if found out that if a variable doesn't exist, NGINX fails to start, printing an error message about the missing variable. The only way to prevent this is by setting unintialized_variable_warn directive is set to off. I can probably enforce this behaviour only for our ingress-nginx build.

Your use case is slightly different, I don't expect any issues. I'll give it some more thought and follow up with you next week.