Default otel_span_attrs can't be overriden

[zguydev]

Describe the bug

The bug is that the default span attrs can't be overriden after they are set.

My case was that I wanted to unset the default nginx span attrs, because they are outdated with modern semconv, which now uses other attr names.
F.e., instead of http.flavor the network.protocol.name should be used, or instead of http.user_agent the user_agent.original should be used and so on.

But after I set the attrs to an empty string (which is the only way to expect the behaviour of unsetting an attr) by using otel_span_attr http.flavor "";, it just adds it as another attr, as seen in Grafana:

To reproduce

Steps to reproduce the behavior:

1. Deploy default nginx using Docker image nginx:1.28.0-alpine-otel with mounted /etc/nginx/conf.d/otel.conf:

# /etc/nginx/conf.d/otel.conf
otel_exporter {
    endpoint  ${X_OTEL_EXPORTER_ENDPOINT};
}

otel_service_name  nginx;

otel_span_attr  http.flavor "";

otel_trace  on;

2. Deploy Grafana with Tempo or any other tracing dashboard.
3. View traces in Grafana, there would be 2 entries of the http.flavor attr.

Expected behavior

It is expected so the http.flavor attr is removed from the span attrs, or so there is at least another way to do so to have full control of the attrs that are set.

Your environment

Docker image: nginx:1.28.0-alpine-otel