Default to DEBUG log level in debug builds (#4749)

### What
* Closes #4629

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[app.rerun.io](https://app.rerun.io/pr/4749/index.html)
* Using examples from latest `main` build:
[app.rerun.io](https://app.rerun.io/pr/4749/index.html?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[app.rerun.io](https://app.rerun.io/pr/4749/index.html?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG

- [PR Build Summary](https://build.rerun.io/pr/4749)
- [Docs
preview](https://rerun.io/preview/86cdb73e839f7f5301f94ccd7b59248b721a7a1e/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/86cdb73e839f7f5301f94ccd7b59248b721a7a1e/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/re_log/src/setup.rs

@@ -5,7 +5,13 @@
 /// Also sets some other log levels on crates that are too loud.
 #[cfg(not(target_arch = "wasm32"))]
 pub fn default_log_filter() -> String {
-    let mut rust_log = std::env::var("RUST_LOG").unwrap_or_else(|_| "info".to_owned());
+    let mut rust_log = std::env::var("RUST_LOG").unwrap_or_else(|_| {
+        if cfg!(debug_assertions) {
+            "debug".to_owned()
+        } else {
+            "info".to_owned()
+        }
+    });
 
     for crate_name in crate::CRATES_AT_ERROR_LEVEL {
         if !rust_log.contains(&format!("{crate_name}=")) {