Validate behaviour of `DirectFileStore` in the presence of existing files

[Sinjo]

We (GoCardless) run our services in containers, which means a clean file system every time we boot the app.

We should look at what the behaviour is like for people who have file systems that persist between versions of the app. If so, we should look at what mitigations we can implement to make DirectFileStore work by default.

Any edge-cases should be added to the DirectFileStore’s docs.

[dmagliola]

See Lawrence's comment on file locking as a potential way of knowing which files are still owned by living processes.

https://github.com/prometheus/client_ruby/issues/143#issuecomment-510068358

Valid warning from Brian that it may not work in all OSes, plus we can't just delete "some" files, but this may be an idea to start building a solution around

[Sinjo]

Similar to #143, and for the same reasons outlined in this comment, I'm taking this out of the v0.10.0 milestone.