docs(all): update & revise docstring examples

[dreamorosi]

Description of your changes

Following the report in #1142 a community member highlighted the fact that some examples in the docstrings of the packages (mainly Tracer) were inconsistent with the ones in the published documentation.

In response to the issue I went through the examples and checked that they were appropriate and consistent, this PR contains the changes related to this review.

Changes:

• Added middleware example to Metrics
• Standardized order of usages in main docstring: middleware, decorator, manual
• Cleaned up imports
• Standardized usage of single quotes

How to verify this change

Check diff.

Related issues, RFCs

Issue number: #1142

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published in downstream module
• The PR title follows the conventional commit semantics

Breaking change checklist

N/A

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[dreamorosi]

Since I have addressed the comments from @flochaz I'm going to merge this PR now instead of waiting for him to click the "approve" button.