feat(ui): simplify flux labels

[sranka]

This PR simplifies labels of flux-based line charts.

Briefly describe your proposed changes:
A label value before this PR

_value[_start=2022-08-17T06:54:02.67759815Z][_stop=2022-08-17T07:54:02.657Z][_field=numSeries][_measurement=database][database=_internal][hostname=66fec3b3194f]

is herein simplified to

numSeries measurement=database database=_internal hostname=66fec3b3194f

with a help of the following rules

• _field value is used in place of _value string, it just shows the field that was measured
• _start, _stop and _field groups keys are removed from the label
• measurement is used in place of _measurement
• • queries that do have no _value and _field columns are also handled properly

What was the problem?

What was the solution?

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass