As g8r once had only a validation webhook, certain metrics
(request_count, request_count_duration_seconds) were implemented without
qualifiers.  i.e., the fact that they applied to the validation webhook
was _assumed_.

As g8r now has both validation and mutation webhooks, separate metrics
for both are required.  These metrics have already been implemented, and
have been released since v3.4.0.  The original metrics were deprecated
(but retained) at the time of that release.

As we are approaching v3.6, it is finally time to remove these metrics.
This PR removes the code and updates the metrics README.

Fixes open-policy-agent#1010

Signed-off-by: juliankatz <juliankatz@google.com>

… deprecate-non-specific-metrics

Signed-off-by: juliankatz <juliankatz@google.com>

… deprecate-non-specific-metrics

Signed-off-by: juliankatz <juliankatz@google.com>

… deprecate-non-specific-metrics

Signed-off-by: juliankatz <juliankatz@google.com>