Fix Rust (1.90?) version

[tigroo]

Just to mention, the collector example code need Rust 1.90 to work, it does not build under 1.85

error[E0658]: `let` expressions in this position are unstable
   --> examples/collector/main.rs:208:32
    |
208 | ...                   if let Some(file_exporter) = &exporter.file
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

error[E0658]: `let` expressions in this position are unstable
   --> examples/collector/main.rs:209:36
    |
209 | ...                   && let Some(ref mut current_log_file) = log_file
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

error[E0658]: `let` expressions in this position are unstable
   --> examples/collector/main.rs:225:36
    |
225 | ...                   && let Some(ref mut current_publish_client) = publish_client
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `libits-client` (example "collector") due to 3 previous errors

Originally posted by @nbuffon in #417 (comment)