Record requests made for provenance tracking

[sckott]

Originally posted by @sckott in https://github.com/ropensci/spocc/issues/189#issuecomment-430418087

moving this note to a new issue.

possibly record requests made somehow so users have a record of what requests were made.

somewhat related to the work in #189 about capturing errors, but instead capture requests made

[sckott]

Q: wonder if its better to:

• collect this metadata and give back in the output of a query, OR
• log to a file that the user can access

i kinda lean towards the latter since then we can collect more metadata to be more useful, w/o bogging down the result objects with more data

[sckott]

Q: should this request provenance tracking be optional?

I think so, since it should add time to the function call. Any downsides to it being optional?