Allow appending custom metadata to each datapoint

[mitar]

Allow appending custom metadata to each datapoint, so that we can store some additional data which should not be really used, but should still be kept to make data more useful for later analysis.

An example is when you measure packet loss by pinging a host with multiple packets, to store how many packets were send to compute packet loss.

This should be stored only on the highest level granularity stream and would not be downsampled.

This would modify append to allow adding this metadata and get_data with a flag to retrieve this metadata (by default false).