Define the sizing rule for Loki volume

[alexandre-allard]

Component: loki, documentation

Why this is needed:
To provide a simple formula to help calculate the ideal size needed for the Loki volume.

What should be done:
Define a formula to calculate needed size for the Loki volume, based on retention, ingestion rate and number of streams.
Test plan for this could be:

• ignore the line-size, just measure its average on a real platform and assume all applications will lead to the same average size
• ignore the retention, just run the benchmarks with a 1 day retention and assume linear scaling (we have a single Loki instance, so that should be roughly true)
• define a synthetic benchmark, to get data points for multiple (ideally random) combinations of #(streams) x rate(ingestion)
• from the gathered data, try to build a linear regression model (with OLS)

Implementation proposal (strongly recommended): n/a

Test plan: n/a