Univariate metrics optimization based on shared fields (attributes, start_time_unix_nano, time_unix_nano)

[lquerel]

Automatically detect data points sharing some or all of their attributes, start_time_unix_nano, or time_unix_nano to minimize redundant data. This optimization doesn't require configuration as opposed to the initial multivariate support proposal.

Note: a native support of real multivariate metrics will be added later.