Enable concurrency for ListMetrics

[justin-watkinson-sp]

While running this exporter in one of my environments, I noticed that the initial spin-up time to do all of the ListMetrics can take some time depending on the namespace. The main reason in my case is that we have a large number of resources under a particular namespace (e.g. AWS/SQS).

To improve performance, I believe we can parallelize these calls by namespace. This should reduce start-up time.

To address API limits, some handling of throttling exceptions and retrying them would be in order. I imagine something along the lines of N goroutines (let's say, 10) are started to call ListMetrics, and configurable via the config file if the implementation wants to raise or lower it.

Thanks!

[frioux]

Seems reasonable to me. We also run into that startup issue, which is why we built it to generally run in a different thread.

[justin-watkinson-sp]

Indeed - been a busy month, but hopefully will get back to some activities such as this in the near future.