added compute metrics by status

[0daryo]

CLOSE #230

% curl http://127.0.0.1:8080/metrics

(...)
fastly_rt_compute_status_code_total{datacenter="GRU",service_id="XXX",service_name="example.com",status_code="200"} 425
(...)
fastly_rt_compute_status_code_total{datacenter="GRU",service_id="XXX",service_name="example.com",status_code="500"} 0

[0daryo]

The metric name compute_status_code_total follows the existing VCL metric naming convention status_code_total, providing consistency across both VCL and Compute platforms.

[leklund]

@0daryo Thank you so much for the contribution. This looks great!