* Alerting: Add metrics to the remote Alertmanager struct
* rephrase http_requests_failed description
* make linter happy
* remove unnecessary metrics
* extract timed client to separate package
* use histogram collector from dskit
* remove weaveworks dependency
* capture metrics for all requests to the remote Alertmanager (both clients)
* use the timed client in the MimirAuthRoundTripper
* HTTPRequestsDuration -> HTTPRequestDuration, clean up mimir client factory function
* refactor
* less git diff
* gauge for last readiness check in seconds
* initialize LastReadinesCheck to 0, tweak metric names and descriptions
* add counters for sync attempts/errors
* last config sync and last state sync timestamps (gauges)
* change latency metric name
* metric for remote Alertmanager mode
* code review comments
* move label constants to metrics package
