Improve the metrics for the proxy service.
* extend the instrumenter middleware to add a label with the name of
the service the request is being dispatched to
* add a native Prometheus histogram that tracks the durations and also
includes a label for the name of the service the request is being
dispatched to, as well as a 'result' label ('success',
'client-error', 'server-error') based on the HTTP status code of the
response
* add per-service gauge functions to count the number of in-flight
requests
* add a counter for routing failures, for when an inbound request
cannot be mapped to a route
* extend the route.RoutingInfo struct with a service attribute, and a
Service() getter
* remove the 'routing_failure_count' metric, as it is redundant
* introduce a fix in the metrics tools introduced with 8821053ff8 in
order to deal with a struct that has fields that are not exported
(as is the case here)
* introduce a func in the toplevel metrics package that registers a
single metric, as that needs to be done explicitly for unexported
ones
* in the graph service, use RegisterAll and BuildInfo from the
toplevel metrics package, as introduced with 8821053ff8
* add the ability to disable the gRPC API handler
(POLICIES_GRPC_DISABLED)
* add the ability to disable the Events API handler
(POLICIES_EVENTS_DISABLED)
* add metrics
* add support for specifying rego files via the environment varirable
POLICIES_ENGINE_FILES
* for file paths specified in yaml or in POLICIES_ENGINE_FILES, support
'config:' and 'data:' path prefixes
* fix typos in the documentation, and try to make it more clear
* add metrics to the documentation
* add a section for testing in the documentation
* introduces a new top-level package pkg/metrics/ with utilities for
metrics that are backported from the groupware branch, with unit
tests
Pascal Bleser