Use event filter (#925)

* Use LevelFilter instead of Level

Originally a `Level` was parsed from one of two environment variables (or
defaulted) and then converted into a `LevelFilter` before initializing the
subscriber.

However, this precludes using `RUST_LOG=off` since `Level` does not
recognize that as valid, resulting in `Level::INFO` (the default) being used.

`LevelFilter` (to which the above is converted anyway) _does_ allow the value to
be `off` - so it seems a little more flexible (and very very minutely faster) to
parse the env var or default value directly into a `LevelFilter`.

* rustfmt

---------

Co-authored-by: Martin Bartlett <jbartle9@dxc.com>

lambda-runtime-api-client/src/tracing.rs

@@ -33,14 +33,15 @@ const DEFAULT_LOG_LEVEL: &str = "INFO";
 pub fn init_default_subscriber() {
     let log_format = env::var("AWS_LAMBDA_LOG_FORMAT").unwrap_or_default();
     let log_level_str = env::var("AWS_LAMBDA_LOG_LEVEL").or_else(|_| env::var("RUST_LOG"));
-    let log_level = Level::from_str(log_level_str.as_deref().unwrap_or(DEFAULT_LOG_LEVEL)).unwrap_or(Level::INFO);
+    let log_level =
+        LevelFilter::from_str(log_level_str.as_deref().unwrap_or(DEFAULT_LOG_LEVEL)).unwrap_or(LevelFilter::INFO);
 
     let collector = tracing_subscriber::fmt()
         .with_target(false)
         .without_time()
         .with_env_filter(
             EnvFilter::builder()
-                .with_default_directive(LevelFilter::from_level(log_level).into())
+                .with_default_directive(log_level.into())
                 .from_env_lossy(),
         );