Force measurement of a metric

[dicksnel]

User story

As a user I want to see the new measurement value for a metric as soon as possible so that I know that the changes I made to the metric or source configuration or changes in the source give the expected results.

Acceptance criteria

• [ ] A user can force Quality-time to measure a metric as soon as possible.

Assumptions

• ~~#7760 is implemented first, otherwise this feature would not provide any feedback to the user about what is happening.~~
• #3134 will imply that they are being measured.

Out of scope

• Showing that Quality-time is currently measuring a metric, see #7760.
• Showing that the config has changed and the current measurement value was measured with the old config, see #3134.

Tasks

Components

Frontend

• [ ] Add button "Measure now" next to the metric position buttons.
• [ ] When the button is clicked, the frontend sets the attribute "next_fetch" of the metric to the current time (no new endpoint is needed).

Documentation

• [ ] Update changelog
• [ ] Add instruction to user manual

[fniessink]

I've created a separate issue for the error message timestamps: #980