Fixes #7261

Hi @mholt, if I may ping you, thanks for labeling the associated issue! I decided to try to contribute a fix, as the scope is quite well defined.

First, I think my initial suggestion in the ticket of using a flat directive would probably
be improved by using a map instead, more consistent to what Caddyfile already does:

tracing {
  span my-span-name
  span_attributes {
    attr1 value1
    attr2 {placeholder}
  }
}

I'm disclosing that I have used Claude to generate most of the code in this PR as this is my first time touching Go in general, although naturally I reviewed the output and tried to understand it. So if holding my hand through the MR is too much, feel free to discard it altogether in favor of a better solution!

If not, there are a couple of things I wanted to ask about. I think given the tests, is pretty already in a reasonable place. Building from that, on a practical level if you have quick pointers:

• Is the use of placeholders implemented and tested reasonably?
• Do we need to do anything special to support JSON configuration? I know we have some JSON processing in the module already in the struct, but I'm asking just in case.
• For documentation, I created WIP: Add span attributes to tracing documentation website#496. Do we need anything more, e.g. in code?

Many thanks! I'm very excited to try out Caddy for real and hopefully switch to it for good.