add missing track_caller to PathRouter method

[Laifsyn]

Motivation

Make internal panics to return the actual offending line instead of redirecting to somewhere inside the library.

pub fn product() -> axum::Router<()> {
    use axum::routing::post;
    axum::Router::new()
        .route("/", post(|| async {}))
        .route("/", post(|| async {}))
         // The resulting message for panic location will display somewhere 
         // internal in the library instead of the user's offendin line.
}

Solution

Add the missing #[track_caller] attribute in the call stack (?)

[Laifsyn]

I embedded a panic in merge_for_path to find what other functions called it yet didn't return the actual callsite at the tests.
So, here's another place that adding the track_caller would allow it to report the callsite of the "offending" test

thread 'routing::tests::merging_routers_with_same_paths_but_different_methods' panicked at axum\src\routing\path_router.rs:198:67: # Inside a protected Method (protected as in `pub(super)`)

[jplatte]

Thanks for bringing this up! I took a look, and I think the better fix is to avoid panicking in internal functions. I implemented that and added a test in #3319.