Add sampleRate option to limit data volume

[jhp]

Limit the addition of traces to engine reports by random sampling.

This is intended to support Glassdoor's use of Apollo server.

[apollo-cla]

@jhp: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[glasser]

Since this PR was opened we've improved usage reporting in several ways such as via the includeReport hook (originally reportTiming) which is very similar to this PR, the ability to aggregate statistics about operations, and to disable field-level instrumentation on a per-operation level. Those improvements should provide outcomes similar or superior to this PR. Sorry for the delay!