Initial approach for adding meta as metrics

[m1keil]

Related to #196

I haven't added anything else beyond the basics. I wonder if this is acceptable approach.

Ideally, I wanted my meta to be exposed like this:

consul_service_meta{service_id="...", node="...", key1="value1", key2="value2"} 1

However, I wasn't familiar with the Prometheus library. It seems like dynamic labels are bit trickier to make, and I thought maybe this is almost as good:

consul_service_meta{service_id="...", node="...", key="key1", value="value1"} 1
consul_service_meta{service_id="...", node="...", key="key2", value="value2"} 1

I've also added metaFilter parameter which enables filtering which meta keys a user would want to expose.

[SuperQ]

This needs a DCO sign-off. You can use git commit -s --amend to add it.

[aholyoake-bc]

@m1keil any chance you you getting this over the line?

[m1keil]

Closing as @aholyoake-bc is leading the effort at: https://github.com/prometheus/consul_exporter/pull/264