Statistics for vcp (connection pools)

Previously, we had zero reporting on the cause of backend connection
errors, which made it close to impossible to diagnose such issues directly.

We now add statistics per connection pool and Debug VSLs.

Ref varnishcache#2622

bin/varnishd/Makefile.am

@@ -237,7 +237,8 @@ VSC_SRC = \
 	VSC_sma.vsc \
 	VSC_smf.vsc \
 	VSC_smu.vsc \
-	VSC_vbe.vsc
+	VSC_vbe.vsc \
+	VSC_vcp.vsc
 
 VSC_GEN_C = @VSC_GEN_C@
 VSC_GEN_H = @VSC_GEN_H@

bin/varnishd/VSC_main.vsc

@@ -758,6 +758,16 @@
 	:oneliner:	ESI parse warnings (unlock)
 
 
+.. varnish_vsc:: vcp_create
+	:level:	diag
+	:oneliner:	Connection Pool creates
+
+
+.. varnish_vsc:: vcp_destroy
+	:level:	diag
+	:oneliner:	Connection Pool destroys
+
+
 .. varnish_vsc:: vmods
 	:type:	gauge
 	:oneliner:	Loaded VMODs

bin/varnishd/VSC_vcp.vsc

@@ -0,0 +1,120 @@
+..
+	This is *NOT* a RST file but the syntax has been chosen so
+	that it may become an RST file at some later date.
+
+.. varnish_vsc_begin::	vcp
+	:oneliner:	Connection Pool Counters
+	:order:		65
+
+
+	Counter for operations on connection pools.
+
+.. varnish_vsc:: n_conn
+	:type:	counter
+	:level:	diag
+	:oneliner:	Number of cached connections
+
+
+.. varnish_vsc:: n_kill
+	:type:	counter
+	:level:	diag
+	:oneliner:	Number of connections still to be closed
+
+
+.. varnish_vsc:: n_used
+	:type:	counter
+	:level:	diag
+	:oneliner:	Number of open connections
+
+
+.. varnish_vsc:: opened
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections opened
+
+	Connections opened successfully
+
+	This counter may be slightly inaccurate for performance reasons.
+
+
+.. varnish_vsc:: closed
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections closed
+
+	Connections closed
+
+
+.. varnish_vsc:: recycle
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections recycled
+
+	Connections kept open (cached) for later reuse
+
+
+.. varnish_vsc:: recycle_fail
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connection recycles (Wait_Enter) failed
+
+
+.. varnish_vsc:: helddown
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connection not attempted due to holddown
+
+	See parameters backend_local_error_holddown and
+	backend_remote_error_holddown.
+
+
+.. varnish_vsc:: fail
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections failed
+
+
+	Counter of failed opens. Detailed reasons are given in the
+	fail_* counters and in Debug VSL.
+
+	This counter is the sum of all detailled fail_* counters.
+
+	All fail_* counters may be slightly inaccurate for efficiency.
+
+.. varnish_vsc:: fail_eacces
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections failed with EACCES or EPERM
+
+
+.. varnish_vsc:: fail_eaddrnotavail
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections failed with EADDRNOTAVAIL
+
+
+.. varnish_vsc:: fail_econnrefused
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections failed with ECONNREFUSED
+
+
+.. varnish_vsc:: fail_enetunreach
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections failed with ENETUNREACH
+
+
+.. varnish_vsc:: fail_etimedout
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections failed ETIMEDOUT
+
+
+.. varnish_vsc:: fail_other
+	:type:	counter
+	:level:	diag
+	:oneliner:	Connections failed for other reason
+
+
+.. varnish_vsc_end::	vcp