[Stopwatch] Allow getting duration of events without calling stop()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [#10175](https://github.com/symfony/symfony/issues/10175)
| License       | MIT
| Doc PR        | [#3539](https://github.com/symfony/symfony-docs/pull/3539)