new option ot_span_decorator

[trashhalo]

use to inspect response and expand attributes.

Anthropic returns usage data in headers in the json response. I wanted that data on the http span. I dont think this a unique ask as rate limits are often represented in headers.

# example from our codebase for anthropic
 ot_span_decorator = fn resp ->
      resp.headers
      |> Enum.filter(fn {k, _v} -> String.starts_with?(k, "anthropic-") end)
      |> Enum.map(fn {k, v} -> {String.replace(k, "-", "."), v} end)
      |> OpenTelemetry.Tracer.set_attributes()

      case resp.body |> Jason.decode!() |> Map.get("usage") do
        nil ->
          :ok

        usage ->
          Enum.each(usage, fn {k, v} ->
            OpenTelemetry.Tracer.set_attribute("anthropic.usage.#{k}", v)
          end)
      end
    end

Then pass to requests at the same level you currently pass ot_attributes