tracing: Headers to tracing (span) attributes

[abolishgenocidenow]

Description:

Do you think it makes sense to have a way to enrich the attributes of each generated span from a specified headers?

For example: If the request has: X-Tracing-Enrichment-User-Region (so operator can specify a prefix, here is X-Tracing-Enrichment) becomes user.region in the span attribute. This is currently very helpful for us to do some tracing export routing. At the tracing export router side, the implementor has an option (read: opportunity) to scrub it before sending it to the actual sink.

[nacx]

This can be already configured, for metrics and traces, with these two controller config values:
https://github.com/envoyproxy/ai-gateway/blob/main/manifests/charts/ai-gateway-helm/values.yaml#L46

This is not accounted for yet in MCP metrics/traces. I'll get that fixed. @mathetake can you assign this to me?