vsl: New Timestamp:Fetch for backend tasks

If significant time is passed in vcl_backend_fetch it becomes difficult
to interpret Timestamp:Connected, which this new timestamp remedies.

bin/varnishd/cache/cache_fetch.c

@@ -420,6 +420,7 @@ vbf_stp_startfetch(struct worker *wrk, struct busyobj *bo)
 	if (wrk->handling == VCL_RET_ERROR)
 		return (F_STP_ERROR);
 
+	VSLb_ts_busyobj(bo, "Fetch", W_TIM_real(wrk));
 	i = VDI_GetHdr(bo);
 
 	now = W_TIM_real(wrk);

bin/varnishtest/tests/c00036.vtc

@@ -20,6 +20,7 @@ varnish v1 -vcl+backend {
 
 logexpect l1 -v v1 -q "vxid == 1004" {
 	expect * 1004 VCL_return      {^fetch}
+	expect 0 1004 Timestamp       {^Fetch:}
 	expect 0 1004 Timestamp       {^Connected:}
 	expect 0 1004 BackendOpen     {^\d+ s1}
 	expect 0 1004 Timestamp       {^Bereq:}

doc/sphinx/reference/vsl.rst

@@ -86,6 +86,9 @@ Backend fetch timestamps
 Start
 	Start of the backend fetch processing.
 
+Fetch
+	Came off vcl_backend_fetch ready to send the backend request.
+
 Connected
 	Successfully established or reused a backend connection.