Change cost_factor to float for rails index

To handle cases where cost factor is between 0 and 1. See https://docs.gitlab.com/ee/ci/pipelines/cicd_minutes.html#cost-factor for more info on how we calculate cost factor for CI minutes.

Example

When looking up log entries for a pipeline (json.pipeline_id: 1234), json.cost_factor is 0, but _source.json.cost_factor is 0.006666666666666667. In this example, the pipeline is for a community contribution MR.