message: subscriber: fix default level for env filter
  ## Motivation
  Setting `RUST_LOG=target` currently enables only the `ERROR` level,
  while it should enable everything.
  ## Solution
  Change the default to `TRACE` if no level is specified
diff: @@ -257,7 +257,8 @@ impl FromStr for Directive {
  let level = caps
  .name("level")
  .and_then(|l| l.as_str().parse().ok())
  - .unwrap_or(LevelFilter::ERROR);
  + // Setting the target without the level enables every level for that target
  + .unwrap_or(LevelFilter::TRACE);
  Ok(Directive {
  level,
  @@ -829,7 +830,7 @@ mod test {
  assert_eq!(dirs[0].in_span, None);
  assert_eq!(dirs[1].target, Some("crate1::mod2".to_string()));
  - assert_eq!(dirs[1].level, LevelFilter::ERROR);
  + assert_eq!(dirs[1].level, LevelFilter::TRACE);
  assert_eq!(dirs[1].in_span, None);
  assert_eq!(dirs[2].target, Some("crate2".to_string()));
  @@ -974,7 +975,7 @@ mod test {
  let dirs = parse_directives("crate1::mod1=wrong,crate2=");
  assert_eq!(dirs.len(), 1, "\nparsed: {:#?}", dirs);
  assert_eq!(dirs[0].target, Some("crate2".to_string()));
  - assert_eq!(dirs[0].level, LevelFilter::ERROR);
  + assert_eq!(dirs[0].level, LevelFilter::TRACE);
  assert_eq!(dirs[0].in_span, None);
  }
  @@ -1001,7 +1002,7 @@ mod test {
  assert_eq!(dirs[0].in_span, Some("foo".to_string()));
  assert_eq!(dirs[1].target, Some("crate1::mod2".to_string()));
  - assert_eq!(dirs[1].level, LevelFilter::ERROR);
  + assert_eq!(dirs[1].level, LevelFilter::TRACE);
  assert_eq!(dirs[1].in_span, Some("bar".to_string()));
  assert_eq!(dirs[2].target, Some("crate2".to_string()));
  