Function otlp_router 

pub fn otlp_router() -> Router

Builds a sub-Router carrying the three OTLP routes plus the body-limit and gzip-decompression layers scoped to those routes.

Layer order, outermost → innermost (request travels through them top to bottom):

1. DefaultBodyLimit::max(300 MiB) — caps the post-decompression bytes the handler’s Bytes extractor will materialize, defending against gzip-bomb expansion that the wire-byte limit can’t see.
2. RequestBodyLimitLayer(20 MiB) — caps the compressed wire bytes; enforced before decompression, returning 413 on oversize.
3. RequestDecompressionLayer — gzip-decodes the body before the handler.
4. handler.