Add examples/system_log_level_overrides.rs

https://github.com/dextero/log-test with its README embedded as a sample
comment.

Suggested on rust-mobile#84.

Author: Marcin Radomski

examples/system_log_level_overrides.rs

@@ -0,0 +1,87 @@
+//! An utility for testing the behavior of `android_logger` crate.
+//!
+//! ## Build
+//!
+//! 1. Setup [`cargo-ndk`](https://github.com/bbqsrc/cargo-ndk)
+//!
+//!    ```
+//!    cargo install cargo-ndk
+//!    rustup target add x86_64-linux-android
+//!    ```
+//!
+//! 2. Build with `cargo ndk`:
+//!
+//!    ```
+//!    ANDROID_NDK_HOME=/usr/lib/android-sdk/ndk/27.1.12297006 \
+//!        cargo ndk -t x86_64 build --release --features android-api-30 \
+//!        --example system_log_level_overrides
+//!    ```
+//!
+//! ## Run on emulator
+//!
+//! 1. Grab a [Cuttlefish](https://source.android.com/docs/devices/cuttlefish)
+//!    virtual device + Android build from [Android
+//!    CI](https://ci.android.com/builds/branches/aosp-main/grid?legacy=1). Select
+//!    the last green `aosp_cf_x86_64_phone` `trunk_staging-userdebug` build and
+//!    open "Artifacts" link, download:
+//!
+//!    - `aosp_cf_x86_64_phone-img-BUILDNUMBER.zip`
+//!    - `cvd-host_package.tar.gz`
+//!
+//! 2. Unpack both archives & start the emulator.
+//!
+//!    ```
+//!    cd $(mktemp -d)
+//!    unzip ~/Downloads/aosp_cf_x86_64_phone-img-*.zip
+//!    tar xf ~/Downloads/cvd-host_package.tar.gz
+//!    HOME=$PWD bin/launch_cvd
+//!    ```
+//!
+//!    Once emulator launches, `adb` should detect it on `0.0.0.0:6520`
+//!    automatically. Shut down the `launch_cvd` command to exit the emulator.
+//!
+//! 3. Upload & run:
+//!
+//!    ```
+//!    adb push ./target/x86_64-linux-android/release/examples/system_log_level_overrides /data/local/tmp/
+//!    adb shell /data/local/tmp/system_log_level_overrides
+//!    ```
+//!
+//! ## Test interaction with Android system properties
+//!
+//! See [`logd`
+//! README](https://cs.android.com/android/platform/superproject/main/+/main:system/logging/logd/README.property)
+//! in AOSP for details.
+//!
+//! ```
+//! # default: should print info+ logs in `adb logcat -s log_test`
+//! # hint: use `adb logcat -v color` is awesome too
+//! adb shell /data/local/tmp/system_log_level_overrides
+//!
+//! # should print trace+ logs in `adb logcat -s log_test`
+//! adb shell setprop log.tag V
+//! adb shell /data/local/tmp/system_log_level_overrides
+//!
+//! # should print warn+ logs in `adb logcat -s log_test`
+//! adb shell setprop log.tag.log_test W
+//! adb shell /data/local/tmp/system_log_level_overrides
+//! ```
+
+fn main() {
+    android_logger::init_once(
+        android_logger::Config::default()
+            .with_tag("log_test")
+            // If set, this is the highest level to log unless overriddeby by the system.
+            // Note the verbosity can be *increased* through system properties.
+            .with_max_level(log::LevelFilter::Info),
+    );
+    // The log crate applies its filtering before we even get to android_logger.
+    // Pass everything down so that Android's liblog can determine the log level instead.
+    log::set_max_level(log::LevelFilter::Trace);
+
+    log::trace!("trace");
+    log::debug!("debug");
+    log::info!("info");
+    log::warn!("warn");
+    log::error!("error");
+}