Cap the number of samples in node-datadog-metrics

https://github.com/dbader/node-datadog-metrics/blob/10d3b29d1fa527807e1301857caac400c54127b6/lib/metrics.js#L133-L136

If ddog is down this will hog a lot of memory.

Start by reading lib/metrics.js at lines 133-136, where samples are accumulated before reporting. Determine how the buffer behaves when Datadog is unavailable, then define and enforce a maximum sample count; done means the buffer cannot grow without limit while preserving the existing reporting behavior.

Written by the indexing model from the issue text.