Reduce cardinality of some of GitLab's Prometheus metrics and fix observed duration reporting.

Closes #41045

See merge request gitlab-org/gitlab-ce!15881

(cherry picked from commit 92e15071)

e391fe1d Reduce cardinality of gitlab_cache_operation_duration_seconds histogram
b02db1f4 Fix gitaly_call_histogram to observe times in seconds correctly
a8ebed60 Make `System.monotonic_time` retun seconds represented by float with microsecond precision
3c545133 Fix tests and formatting
ed715b79 use in_milliseconds rails helper
040167f0 Use seconds where possible, and convert to milliseconds for Influxdb consumption
10af36f0 add missing word to pawel-reduce_cardinality_of_prometheus_metrics.yml