[prometheus-to-sd] Add quantiles to summary type

[AttwellBrian]

As discussed in #196, summary quantiles cannot be aggregated well. However, they are still useful and frequently used by Prometheus exporters. In order to allow stackdriver to be more of a drop in replacement for prom, this diff adds support for summary quantiles

Each quantile acts like a seperate label value. For the gauge.

Test Plan
Modified translater_test.go. Temporarily added the following to the Dockerfile. Then ran the Dockerfile.

RUN file="$(CGO_ENABLED=0 GOOS=linux go test ./translator/)"; echo $file

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[AttwellBrian]

This is a seperate issue that I'll look into in a separate diff.

I've seen this cause some frustrating issues before, outside of summary metric types.

[AttwellBrian]

verify-gofmt.sh returns no problems on my machine. So I'm not certain what to do here.