Varnish 3 (#51)

* Test the Prometheus gather
The "gauge should be a Counter" error doesn't present until the Prometheus module attempts to gather the metrics for display (see https://github.com/prometheus/client_golang/blob/4efc3ccc7a660996803fff4721a2643b33dc3f77/prometheus/registry.go#L610).

This test runs through the test varnishstat files and collects them into a Prometheus registry and then calls `Gather()` to ensure there are no errors.

* Add test for varnish 3 scraping and gathering

* Remove the `main_n_ban` grouping.
This is no longer used in Varnish 4+ as the metric was renamed to `n_bans` rather than `n_ban`. And in Varnish 3 it breaks as the `n_ban` and `n_ban_gone` are `i` (Integer) flags while all the other `n_ban*` metrics are `a` (Accumulator) flags. (See https://github.com/varnishcache/varnish-cache/blob/3.0/include/vsc_fields.h#L142-L148)

* Revert accidental update to go.mod

prometheus.go

@@ -165,13 +165,6 @@ var (
 			total:     "main_n_wrk",
 			desc:      "Number of worker threads",
 		},
-		&grouping{
-			newPrefix: "main_bans",
-			prefix:    "main_n_ban",
-			total:     "main_n_ban",
-			desc:      "Number of bans",
-			labelKey:  "action",
-		},
 	}
 	fqNames = map[string]string{
 		"varnish_lck_colls":   "varnish_lock_collisions",