add loki resource usage dashboard for read and write path

[sandeepsukhani]

What this PR does / why we need it:
These dashboards are inspired by Reads and Writes resource usage Cortex dashboards.
They come in handy when we want to quickly find out bottlenecks in reads/writes or whether we have over-provisioned or under-provisioned the cluster.

Here are the screen captures showing how dashboards would look

Reads Resources:

Writes Resources:

[dannykopping]

Looks awesome @sandeepsukhani 🎉