impl(o11y): box futures in DiscoveryPoller to prevent stack overflow

[haphungw]

When tracing is enabled, DiscoveryPoller is wrapped inside nesting decorators. Because it uses generic type parameters S (start future) and Q (query future), the compiler generates extremely deeply nested type definitions for every generic type combination.

At runtime, executing these deeply nested future types requires a lot of stack memory, resulting in a stack overflow in LRO-heavy tests (compute::run_compute_lro_errors).

Therefore, instead of keeping the futures as generic type parameters, we store them as boxed futures on the heap (Pin<Box<dyn Future + Send>>). This erases their concrete types and flattens their size on the stack.

[gemini-code-assist]

Code Review

This pull request refactors DiscoveryPoller to use boxed futures (BoxedFuture) instead of generic parameters S and Q, simplifying the type signatures and trait implementations. The feedback suggests shadowing the query parameter instead of naming it query_clone to avoid confusion, as the variable is moved into the closure rather than cloned.

[gemini-code-assist]

The variable is named query_clone, but it is actually moved into the closure rather than cloned. To avoid confusion and improve readability, you can simply shadow the query parameter.

Suggested change

	let mut query_clone = query;
	let start_boxed = Box::pin(start());
	let query_boxed = Box::new(move |name| Box::pin(query_clone(name)) as BoxedFuture<Result<O>>);
	let mut query = query;
	let start_boxed = Box::pin(start());
	let query_boxed = Box::new(move |name| Box::pin(query(name)) as BoxedFuture<Result<O>>);

References

1. Encourage modern, efficient, and readable Rust code. ^(link)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.90%. Comparing base (737706a) to head (45840d8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5918   +/-   ##
=======================================
  Coverage   97.90%   97.90%           
=======================================
  Files         234      234           
  Lines       59451    59458    +7     
=======================================
+ Hits        58204    58212    +8     
+ Misses       1247     1246    -1     

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