Support for varnish 6.3.1

[akshatgit]

• Varnish 6.3.1
• prometheus_varnish_exporter 1.5.2

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The current version is giving the following error:

* collected metric "varnish_backend_fail_econnrefused" { label:<name:"backend" value:"xxx" > label:<name:"server" value:"unknown" > counter:<value:0 > } was collected before with the same name and label values
* collected metric "varnish_backend_fail_enetunreach" { label:<name:"backend" value:"xxx" > label:<name:"server" value:"unknown" > counter:<value:0 > } was collected before with the same name and label values
* collected metric "varnish_backend_beresp_bodybytes" { label:<name:"backend" value:"xxx" > label:<name:"server" value:"unknown" > counter:<value:1.4573694677e+10 > } was collected before with the same name and label values
* collected metric "varnish_backend_unhealthy" { label:<name:"backend" value:"xxx" > label:<name:"server" value:"unknown" > counter:<value:0 > } was collected before with the same name and label values

[krazy350]

Check if code from master works. Its probably caused by #57

[hashworks]

I just tested with 7.5.0, works fine.