refactor(tracing): refactor internal code and improve docs

[lcian]

• Refactor some code in preparation for the logs integration
• No breaking changes
• Improve the docs a little
• Test that a breadcrumb with errors carries the stringified error chain, behavior we added in refactor(tracing): remove EventFilter::exception and always attach exception #768
• It's also a behavioral change because we were previously attaching all error fields in an errors field in the JSON data of the breadcrumb, which doesn't make much sense. Now we'll store them in separate fields, retaining the original name

example:

            tracing::info!(
                database_error = &db_error as &dyn Error,
                parse_error = &parse_error as &dyn Error,
                retry_count = 3,
                connection_timeout = "30s",
                "Multiple errors occurred during database operation"
            );

Before:

After:

[lcian]

As of this PR, store_errors_in_values = true will be used for breadcrumbs.
We will use the same behavior for logs, so instead of duplicating code we make it a flag here on the visitor

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.40%. Comparing base (2b1455b) to head (97b7361).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #839      +/-   ##
==========================================
+ Coverage   73.36%   73.40%   +0.03%     
==========================================
  Files          64       64              
  Lines        7678     7682       +4     
==========================================
+ Hits         5633     5639       +6     
+ Misses       2045     2043       -2     

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