Add metric for loop detection

[mtorluemke]

We should have a metric that increments when a proxy loop is detected. Likely here:

trafficserver/proxy/http/HttpTransact.cc

Line 6577 in f36cf6a

SET_VIA_STRING(VIA_ERROR_TYPE, VIA_ERROR_LOOP_DETECTED);

[bryancall]

@cmcfarlen will take a look at this.