improve observability

[twosigmajab]

It would be useful to record basic events and metrics (cache size, hits, misses, evictions, etc.). I took a look through the docs and the code and didn't see anything about this. Even just logging these events when they occur at debug level would be a big improvement, to give users the option of having some more visibility into what the cache is actually doing. What do you think?

P.S. Thanks for maintaining Flask-Caching!

[sh4nks]

While I think having such a feature might be helpful for debugging I have no plans implementing such feature.

[twosigmajab]

The minimum required here is just a matter of adding a few logger.debug calls. Would you at least accept a PR?

[sh4nks]

Yeah, I'd accept a PR :-)

[jab]

Thanks for considering a PR for this, @sh4nks! Please see #203. Hope this is helpful!