docs(observed): add event sampling example

[Evgeny Fedorov (evgenyfedorov2)]

• move the example
• add a changelog entry

[Copilot]

🟡 Changes recommended

The Unreleased changelog entry is misleading relative to the PR (it describes an API “initial release” rather than the added/moved documentation/example).

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Pull request overview

This PR improves observed’s documentation around per-Sink event sampling by moving the inline sampler example out of API docs into a dedicated runnable example, and updating crate-level docs/README to mention the feature.

Changes:

• Removed the long inline with_event_sampler/sampling module doctest examples and replaced them with a link to a new event_sampling example.
• Added a new runnable example crates/observed/examples/event_sampling.rs demonstrating EventSampler usage.
• Added an Unreleased changelog entry (currently worded as an API “initial release”).

File summaries

File	Description
crates/observed/src/sink/core.rs	Replaces the inline sampler example with a shorter “Sharing” explanation for sampler configuration across clones.
crates/observed/src/sampling/mod.rs	Removes module-level doctest example and links to the new event_sampling example from EventSampler docs.
crates/observed/src/lib.rs	Mentions per-Sink event sampling in the crate feature list.
crates/observed/README.md	Regenerated README to include the new sampling bullet and updated reference links.
crates/observed/examples/event_sampling.rs	Adds a dedicated runnable example showing event sampling while preserving borrowed event data.
crates/observed/CHANGELOG.md	Adds an Unreleased entry related to event sampling (needs rewording to match PR scope).

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 1
• Review effort level: Lite

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (40c705c) to head (7c463c0).

Additional details and impacted files

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  Coverage   100.0%   100.0%           
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  Files         583      583           
  Lines       62891    62891           
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  Hits        62891    62891           

Flag	Coverage Δ
linux	68.9% <ø> (?)
linux-arm	68.9% <ø> (?)
windows	63.8% <ø> (?)

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[Copilot]

🟢 Approval recommended

Changes are documentation/example-focused and appear consistent with existing repository patterns for linking to examples.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0 new
• Review effort level: Lite