NQE: Add metric that can be used for computing correlation between low level metrics (e.g., transport RTT estimate, HTTP RTT estimate, bandwidth estimate) and high level metrics (e.g., time taken to fetch resource, page load time).

This will help in determining which low-level metrics best capture the network performance, and if the computed estimates (e.g., bandwidth estimate) actually reflect the network performance.

In the future, this can also be used to determine the additional low level metrics that NQE can take into account while estimating the network performance. e.g., recording the signal strength, and resource fetch time, and then computing their correlation can tell us how useful signal strength is in determining the network performance.