Heavy memory usage

[danielmbarlow]

We deployed OpenCensus with the Stackdriver exporter to production and found that it used a lot more memory. Specifically our app went from using 3-4GB RAM to around 12GB.

We disabled the exporter and the memory issue disappeared so I suspect the issue comes from the opencensus-ruby-exporter-stackdriver gem.

Another PR suggests that this may be related to the use of the concurrent-ruby gem. @luong-komorebi if you could shed any light on what to look for in fixing the memory usage issue perhaps it would help the project maintainers.