chore: extend RouteProvider trait with routes_stats()

[nikolay-komarevskiy]

Description

RouteProvider trait is extended with a method to get useful metrics about the total number of routes and healthy number of routes.

/// Returns the total number of routes and healthy routes as a tuple.
///
/// - First element is the total number of routes available (both healthy and unhealthy)
/// - Second element is the number of currently healthy routes, or None if health status information is unavailable
///
/// A healthy route is one that is available and ready to receive traffic.
/// The specific criteria for what constitutes a "healthy" route is implementation dependent.
fn routes_stats(&self) -> (usize, Option<usize>);

How Has This Been Tested?

Unit tests were extended accordingly.

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.