otel_span_attr from different levels are not merged

[danielkatz]

Describe the bug

When otel_span_attr directives are declared at both the http level and the server (or location) level, only one group of attributes is reported in the span—not both. The attributes are not merged; instead, the child context's attributes completely replace the parent's.

To reproduce

1. Configure nginx with otel_span_attr at both http and server levels:

http {
    otel_exporter {
        endpoint localhost:4317;
    }

    otel_trace on;
    otel_span_attr http.level "from-http";

    server {
        listen 8080;
        otel_span_attr server.level "from-server";

        location / {
            return 200 "ok";
        }
    }
}

2. Send a request to the server
3. Inspect the exported span attributes

Actual result: Only server.level attribute is present.

Expected behavior

Both http.level and server.level attributes should be present in the span. Attributes declared at different configuration levels should be merged, with child levels potentially overriding parent values for the same attribute name.

Your environment

• Version/release: nginx/1.29.4
• Target deployment platform: container base on alpine:3.22

Additional context

The issue is in mergeLocationConf in src/http_module.cpp:

if (conf->spanAttrs.elts == NULL) {
    conf->spanAttrs = prev->spanAttrs;
}

This logic only inherits parent attributes when the child has none. If the child has any otel_span_attr directives, the parent's attributes are silently discarded rather than merged.

[danielkatz]

Here is a full reproduction of this behavior: https://github.com/danielkatz/nginx-otel/tree/otel_span_attr_merge/reproduce

[danielkatz]

What should be the correct implementation?

1. Only otel_span_attrs from the most specific scope have effect (as it is now)
2. otel_span_attrs from all levels are merged
   a. In case of duplicate key, most specific wins
   b. In case of duplicate key, all configured attrs appear (i think otel will use the last)

The docs don't mention this at all.

[danielkatz]

I implemented a fix in PR #108. I chose option 2b (merge without deduplication) to match the current behavior when the same key is set twice at the same level.

When you declare otel_span_attr my.key "value1" and otel_span_attr my.key "value2" in the same block, both values are included. The fix merges attributes across levels the same way—parent attributes first, then child attributes. If the same key exists at multiple levels, both values appear in the span, and the collector handles deduplication.