feat(metrics): add delegated routing client metrics to prometheus

[hsanjuan]

Added exporting of routing http client metrics: the endpoint will now include someguy_routing_http_client_* metrics when delegated routing clients are used:

• Histogram: the latency of operations by the routing HTTP client:
  • someguy_routing_http_client_latency_bucket{code,error,host,operation,le}
  • someguy_routing_http_client_latency_sum{code,error,host,operation}
  • someguy_routing_http_client_latency_count{code,error,host,operation}
• Histogram: the number of elements in a response collection
  • someguy_routing_http_client_length_bucket{host,operation,le}
  • someguy_routing_http_client_length_sum{host,operation}
  • someguy_routing_http_client_length_count{host,operation}