Instrument Rails cache code

See merge request !3619

Update gitlab-shell to 2.7.2

See merge request !3594

Use meta key to open todo in new tab

Fixes #15078 

See merge request !3623

Remove TODO for not documented stuff

We should not put a `TODO` in a released product.

See merge request !3553

First pass at a Testing styleguide

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14121

[ci skip]

See merge request !3465

Use more accurate timestamps for InfluxDB.

See merge request !3617

This allows us to track how much time of a transaction is spent in
dealing with cached data.

Use gitlab-workhorse 0.7.2

See merge request !3570

This changes the timestamp of metrics to be more accurate/unique by
using Time#to_f combined with a small random jitter value. This
combination hopefully reduces the amount of collisions, though there's
no way to fully prevent any from occurring.

Fixes gitlab-com/operations#175

API: Expose subscribed? on issues and merge requests

This is the first start for the new `subscribable` API. It exposes the `subscribed` field for issues and merge requests.

Related to #6024

See merge request !3611

Fix Incorrect Quote In Docker Executor Example

## What does this MR do?
The corrected line opened with a regular quote (') but closed with a back-tick (`)  
I believe this is invalid bash syntax

## Are there points in the code the reviewer needs to double check?
Probably not

## Why was this MR needed?
Fixes error when example is copied and pasted

## What are the relevant issue numbers?
None

## Screenshots (if relevant)
None

See merge request !3604

[ci skip]

This reverts merge request !3494

This reverts merge request !3520

This commit attempts to change default user search scope if otp_user_id
session variable has been set. If it is present, it means that user has
2FA enabled, and has already been verified with login and password. In
this case we should look for user with otp_user_id first, before picking
it up by login.

Closes #14961

This also contains specs for a bug described in #14900