[metrics/cloudwatch] Feature request: ability to publish a sample of metrics for histograms

The [cloudwatch metrics exporter](https://github.com/go-kit/kit/tree/master/metrics/cloudwatch) currently has support for publishing histograms as a set of metrics of the histogram quantiles (e.g. p50, p90, p95, p99).

What I would like to do, is have the ability to instead publish a random sample of the recorded histogram values and let CloudWatch do the bucketing. This is because I have multiple instances that I would like to publish the same metric for without labeling them with a label unique to the instance. If there are less than 150 observations (the max for an individual `PutMetricsData` request to AWS), they would all be sent.

I pushed some changes up to demonstrate what I mean: https://github.com/go-kit/kit/compare/master...jszwedko:metrics/cloudwatch/add-ability-to-publish-histogram-values?expand=1 . If you all are amenable to this change, I can continue implementing it and open a PR.

Thanks!