* Fix various read only storage errors
A mistake we've seen multiple times in our own plugins and that we've
seen in the GCP plugin now is that control flow (how the code is
structured, helper functions, etc.) can obfuscate whether an error came
from storage or some other Vault-core location (in which case likely it
needs to be a 5XX message) or because of user input (thus 4XX). Error
handling for functions therefore often ends up always treating errors as
either user related or internal.
When the error is logical.ErrReadOnly this means that treating errors as
user errors skips the check that triggers forwarding, instead returning
a read only view error to the user.
While it's obviously more correct to fix that code, it's not always
immediately apparent to reviewers or fixers what the issue is and fixing
it when it's found both requires someone to hit the problem and report
it (thus exposing bugs to users) and selective targeted refactoring that
only helps that one specific case.
If instead we check whether the logical.Response is an error and, if so,
whether it contains the error value, we work around this in all of these
cases automatically. It feels hacky since it's a coding mistake, but
it's one we've made too multiple times, and avoiding bugs altogether is
better for our users.
Increment a counter whenever a request is received.
The in-memory counter is persisted to counters/requests/YYYY/MM.
When the month wraps around, we reset the in-memory counter to
zero.
Add an endpoint for querying the request counters across all time.
