Improve metrics processing and aggregation

[yorugac]

The extension must send samples with values that would be understandable to remote agent. To achieve that, k6 aggregation methods of Sink interface are used. This leads to the following problems:

1. Sink's Add and Format methods are not very efficient; additionally, Trend's Sink is not fit for such usage at all so ad-hoc optimizations must be used as is done here (related k6 issue)
2. attempt to group samples for each metric into TimeSeries leads to losing data in tags / labels (related k6 issue)
3. converting all samples into separate TimeSeries results, at best, in hitting early whatever usage limit is currently set at Prometheus

Since adding any kind of further pre-processing here might clash with the future work on Metric Refactoring at k6, improving these can be considered blocked for now.