browser.formatters.level does not inline attributes into message anymore

[timtrense-leadec]

after trying out the sample code of PR#1898 with v9.0.0 I sadly must report that the level formatter for browser does not inline the contained attributes anymore, but blindly puts a level object in the resulting log entry.

Expected:

{
  time: 1707198967745,
  label: 'info',
  level: 30,
  msg: 'I am logging a message from the middleware'
}

Actual:

{
  time: 1707198967745,
  level: {label: 'info', level: 30},
  msg: 'I am logging a message from the middleware'
}

[timtrense-leadec]

This seems to be a degradation after v.8.19.0 in 7f33a0e

In deleted line 339 there used to be an Object.assign, whereas now in added line 338 is a simple assignment of o.level = levelFormatter(...).

[emmyakin]

This changes causing this bug has been reverted in #1971, and thus fixed