docs: refactor "Custom Middleware" guide

[sobolevn]

Link: https://docs.litestar.dev/latest/usage/middleware/creating-middleware.html

Changes:

• I removed super().__init__ call for Protocol, it should not happen by definition, because Protocol cannot have any implementation in __init__
• I used time.monotonic for timing examples, because it is more fitting for the job, I can use perf_counter instead if you wish
• I removed several if TYPE_CHECKING blocks, because they made examples more complex to read with no real need
• logger.info should be lazy in terms of string format, not eager
• ~ usage, where possible
• exclude_from_middleware -> exclude_from_my_middleware, because it was not clear if this name was special or not. Now it is clear that it is a user defined name.

And other small fixes here and there :)

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/3833