log cache usage (or attach it to spans)

[butonic]

When running in production and for load tests it would be great to see if there was a cache hit or miss.

Maybe we can increase a metrics counter for cache hit/miss when reading directories and file metadata as well as user metadata.

And finally a metric how often the share provider had to retry writing the metadata to the system storage. Under load this seems to take ages.

cc @fschade