Make the schema for BQ tables storing metric results more generic (Python)

Currently, we keep the metrics results in BQ in tables with a schema like this: 

timestamp | total_bytes | run_time | (possibly other BQ columns)

every time we want to add a new column the schema has to be extended. This is not convenient given the fact that any load test can have different metrics stored. This in turn would cause multiple BQ tables each queried differently. 

We can provide a more generic schema, like so: 

test_id | timestamp | metric | value

thanks to that, every metric, whatever it's name is, can be saved in the table as a separate row. This gives more elasticity in storing metrics and is still easy to query and plot.