Introduce a new metric for calculating memory usage percentage

[tsehori]

Currently, all GRR server components expose two Prometheus metrics which are related to memory usage: process_virtual_memory_bytes (Virtual memory size in bytes) and process_resident_memory_bytes (Resident memory size in bytes). Both metrics are implemented as Gauges, and are exposed by the Prometheus client library in GRR. Further explanation of these two metrics can be found in the official documentation of writing client libraries for Prometheus.

We suggest introducing a new metric, such as current memory used in bytes, that will allow calculating and graphing memory usage percentage. Using this metric, we can create a relevant graph, which can be implemented as part of the grr_grafanalib_dashboards module and inserted into the existing sample dashboards for GRR servers.
Moreover, such a metric can be used for calculations other than memory usage percentage; ideas are welcome!