Make large MI measurement logs more compact

[okt-viacheslav]

MI measurements are logged as separate JSON objects, each duplicating the measurement aggregation type, units and multiplier. This makes each measurement take much more space than a single floating point value.

Introduce a new "series" aggregation type that allows tests to log a series of measurements that share their properties in one JSON object.

For example, the following sequence

{"aggr":"single","value":0.00202225,"base_units":"seconds","multiplier":"1"}
{"aggr":"single","value":0.00303048,"base_units":"seconds","multiplier":"1"}
{"aggr":"single","value":0.00403947, "base_units":"seconds","multiplier":"1"}

turns into one object:

{"aggr":"series","values":[0.00202225, 0.00303048, 0.00403947],"base_units":"seconds","multiplier":"1"}

Our test suite provides test iterations that can log 30000 such measurements, see below for the final log sizes before these patches, after compacting MI log messages, and after removing 1.0 multipliers from HTML logs:

~/run-old$ du -sh tmp_raw_log log.txt html
854M    tmp_raw_log
395M    log.txt
1.3G    html

~/run-compact$ du -sh tmp_raw_log log.txt html
292M    tmp_raw_log
398M    log.txt
768M    html

run-compact-html$ du -sh tmp_raw_log log.txt html
287M    tmp_raw_log
392M    log.txt
352M    html