Support meaningful instance labels

[candlerb]

When using meaningful instance labels there is only a name, and no port number, in the instance label. Unfortunately, the existing node_exporter dashboard does not work without a port.

I suggest using the instance label directly.

Also, given that job and node are not multi-select, these can use = rather than =~ for matching (more efficient, less likely to trip up on regexp metacharacters)

I've made these changes in 12486 which is a direct fork of 1860.

[rfrail3]

Thanks for update the dashboard with the work done, it helps.