Adds a `trace.Tracer` to the `cache.Cache`so
that all operations can be traced. Also, updates
the `utils.FnCache` to link the `context.Context`
passed to the loadFn to any current spans. This
will allow any fetches made by the `FnCache` to
be associated with the call that lead to loading.
Note: There are a few methods in the `auth.Cache`
interface which do not take a `context.Context`.
For the time being all spans for these calls use
a `context.TODO` and will be updated in the future,
where the changes can be made on a per method basis.
rosstimothy
14ebab8b96