Count proxy.process.http.incoming_requests at transaction start

[bneradt]

The proxy.process.http.incoming_requests metric was only incremented in HttpTransact::HandleRequest(), which is called late in the request processing pipeline. Requests that received error or redirect responses before reaching HandleRequest were never counted, causing significant underreporting during high-error-rate scenarios.

This fix moves the increment to HttpSM::attach_client_session(), which is called at the start of every transaction. This ensures all requests are counted, including those that fail header parsing, receive remap redirects, or hit early error conditions. This aligns with the documented behavior that incoming_requests should include errors and redirects (see doc/appendices/command-line/traffic_top.en.rst):

"Total number of client requests serviced by Traffic Server. This
includes both successful content-bearing responses as well as
errors, redirects, and not-modified IMS responses."

[maskit]

The new location looks better, but I'm wondering if this is the earliest possible point to increment the metric. I think H2 requests that have decoding errors (e.g. unrecognized pseudo headers, compression errors, etc) are still not counted.