Use timestamp deltas in grafana-data-store (#4199)

* Optimize grafana storage slightly

* Test on grafana test binary

* Cast to f32

* Silence record_metrics warning

* Add source for error

* More changes

* Fix record_metrics_slice

* Test Database

* Docs

* Update client/grafana-data-source/src/lib.rs

Co-Authored-By: Niklas Adolfsson <niklasadolfsson1@gmail.com>

* Fix macro

* Update server.rs