message: subscriber: fix extra padding in pretty format
  ## Motivation
  This fixes where extra padding was written when logging with the
  pretty formatter.
  ## Solution
  With this change we only write padding when we actually decide to write
  a value but not when skipping a metadata value such as `log.file` or
  `log.line`
diff: @@ -241,12 +241,14 @@ impl<'a> PrettyVisitor<'a> {
  Self { style, ..self }
  }
  - fn maybe_pad(&mut self) {
  - if self.is_empty {
  + fn write_padded(&mut self, value: &impl fmt::Debug) {
  + let padding = if self.is_empty {
  self.is_empty = false;
  + ""
  } else {
  - self.result = write!(self.writer, ", ");
  - }
  + ", "
  + };
  + self.result = write!(self.writer, "{}{:?}", padding, value);
  }
  }
  @@ -287,28 +289,25 @@ impl<'a> field::Visit for PrettyVisitor<'a> {
  return;
  }
  let bold = self.style.bold();
  - self.maybe_pad();
  - self.result = match field.name() {
  - "message" => write!(self.writer, "{}{:?}", self.style.prefix(), value,),
  + match field.name() {
  + "message" => self.write_padded(&format_args!("{}{:?}", self.style.prefix(), value,)),
  // Skip fields that are actually log metadata that have already been handled
  #[cfg(feature = "tracing-log")]
  - name if name.starts_with("log.") => Ok(()),
  - name if name.starts_with("r#") => write!(
  - self.writer,
  + name if name.starts_with("log.") => self.result = Ok(()),
  + name if name.starts_with("r#") => self.write_padded(&format_args!(
  "{}{}{}: {:?}",
  bold.prefix(),
  &name[2..],
  bold.infix(self.style),
  value
  - ),
  - name => write!(
  - self.writer,
  + )),
  + name => self.write_padded(&format_args!(
  "{}{}{}: {:?}",
  bold.prefix(),
  name,
  bold.infix(self.style),
  value
  - ),
  + )),
  };
  }
  }
  