Add cost warning and filtering example to CloudWatch documentation

[hong-kyungsik]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

This PR enhances the CloudWatch metrics documentation by adding guidance about potential cost implications and metric filtering.

Specifically, it adds:

• A warning that enabling CloudWatch metrics can lead to unexpected costs
• An explanation that CloudWatch charges per metric name + dimensions
• A recommendation to use Micrometer MeterFilter to limit exported metrics
• A concrete example showing how to allow only a selected set of metrics
• This change is documentation-only and does not alter any existing behavior.

💡 Motivation and Context

When CloudWatch metrics integration is enabled, Spring Boot Actuator and Micrometer publish a large number of default metrics.
Without explicit filtering, this may result in unintended CloudWatch costs.

Currently, the documentation does not clearly explain:

• The cost model of CloudWatch custom metrics
• How to control which metrics are exported

This PR addresses that gap by making the cost implications explicit and documenting a recommended way to restrict exported metrics.

While this PR does not change the default behavior, it helps clarify and mitigate the concerns raised in #873 by documenting the cost implications and recommended configuration.

The example code included in this documentation is based on the snippet originally shared by the issue author

💚 How did you test it?

• Verified locally that a MeterFilter.denyUnless configuration allows only the specified metric names to pass through
• Confirmed that non-whitelisted metrics are filtered out at the Micrometer level
• No changes to runtime behavior were introduced

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated reference documentation to reflect the change
• All tests passing
• No breaking changes

🔮 Next steps

[maciejwalkowiak]

Thank you!