Move Top Stats to new internal stats endpoint

[RobertJoonas]

Changes

This PR introduces a new POST /api/stats/:domain/query endpoint and makes the Top Stats use it. The endpoint will eventually replace all the Api.StatsController actions such as main_graph, sources, pages, etc.

• To call the new endpoint, there's a new stats function in api.ts. It takes a StatsQuery object which aims to mimic the query params required to construct %ParsedQueryParams{} on the backend.
• In order to build the StatsQuery object, there's also a new function called createStatsQuery which takes DashboardState and ReportParams objects as arguments. ReportParams contains the information about a specific report, e.g.: dimensions and metrics.

While this PR does not intend to change any behaviour, there are some functional changes with this move:

1. bounce_rate is now shown in page-filtered Top Stats, even when imports are included in the view. Since bounce rate from imported data is unavailable in this view, there's an appropriate warning, like with scroll depth. (This was actually already handled by Include bounce rate in top stats with page filter and imported data + fix 500 #6073, but listing it here for better visibility as well)
2. time_on_page is now shown in page-filtered Top Stats, even when imports are included in the view. There's a specific exception where the metric is dropped on the backend though -- when legacy time on page is queried in aggregate from imported data. That's because the functionality in the Legacy.TimeOnPage module does not exist. This was likely the reason why time on page was simply cut out of page-filtered Top Stats when imports are included.
3. Top Stats time labels (the ones rendered only in comparison mode) do not span into the future anymore. E.g. if today is Feb 2nd, then:

Before:

After:

Tests

• Automated tests have been added

Changelog

• This PR does not make a user-facing change

Documentation

• This change does not need a documentation update

Dark mode

• This PR does not change the UI

[github-actions]

Preview environment👷🏼‍♀️🏗️
PR-6035

[apata]

Works well!

[zoldar]

@RobertJoonas I have rebased this branch against the (pending) e2e test suite and I have stumbled on one issue:

When switching to realtime, it appears that top stats metrics IDs for current visitors and visitors from the last 30 minutes have the same ID ("visitors"). Before the change, "Current visitors" metrics has a distinct ID of "current_visitors".

[RobertJoonas]

@RobertJoonas I have rebased this branch against the (pending) e2e test suite and I have stumbled on one issue:

@zoldar, I have addressed this with a hotfix for now. abcdabc.

Planning to drop the separation of topStatsQuery and currentVisitorsQuery in a follow-up. The current visitors could just be an integer value in the formatted Top Stats structure. E.g: {topStats: [...], currentVisitors: integer | null}

[zoldar]

@RobertJoonas nice SHA :D