Add _meta tag containing the number of breadcrumbs truncated

[shellmayr]

Problem Statement

• The Python SDK currently does not indicate when breadcrumbs are truncated due to the number of breadcrumbs exceeding the default limit or the limit specified in the sentry_sdk.init call. This information is however needed in order to surface this information in the Sentry UI.

• The motivation and context are outlined in https://github.com/getsentry/projects/issues/593 and its parent https://github.com/getsentry/projects/issues/396.

Solution Brainstorm

Add a _meta entry using an AnnotatedValue to indicate whenever breadcrumbs were truncated. The _meta entry should include the number of breadcrumbs that were removed.

This requires the SDK to

1. keep track of the number of breadcrumbs truncated,
2. and when an error is captured, record that number into the event _meta.