fix: Escape non-printable characters in logfmt

The is no specification logfmt, most implementations only unescape `\"` and leave
`\n` as is when parsing. Some implementations like go-logfmt do escape newlines as `\n` (and other chars
according to Go's escaping rules).

This PR just escapes non-printable characters using rust's string literal rules; they are not perfect but
they don't seem to be worse than other players.

(If this bites us some more we will probably be better of switching to json log output format and then equip ourselves with some simple shell scripts to reformat the logs on the fly while we're streaming them from k8s)

As a bonus point, this effectively simplifies the escaping code, possibly also speeding it up a bit
since it doesn't have to call replace twice (although I don't know what magic would the rust compiler have done to this double replace).

Closes #1791
Closes influxdata/k8s-iox#1

logfmt/src/lib.rs

@@ -281,16 +281,15 @@ fn needs_quotes_and_escaping(value: &str) -> bool {
         return true;
     }
 
-    value.contains(' ') && !pre_quoted
+    let has_not_printable = value.bytes().any(|b| b <= b' ');
+
+    has_not_printable && !pre_quoted
 }
 
 /// escape any characters in name as needed, otherwise return string as is
 fn quote_and_escape(value: &'_ str) -> Cow<'_, str> {
     if needs_quotes_and_escaping(value) {
-        Cow::Owned(format!(
-            "\"{}\"",
-            value.replace(r#"\"#, r#"\\"#).replace(r#"""#, r#"\""#)
-        ))
+        Cow::Owned(format!("{:?}", value))
     } else {
         Cow::Borrowed(value)
     }
@@ -376,4 +375,16 @@ mod test {
             r#""a \"0 \\\"1\\\" 2\" c""#
         );
     }
+
+    #[test]
+    fn quote_not_printable() {
+        assert_eq!(quote_and_escape("foo\nbar"), r#""foo\nbar""#);
+        assert_eq!(quote_and_escape("foo\r\nbar"), r#""foo\r\nbar""#);
+        assert_eq!(quote_and_escape("foo\0bar"), r#""foo\u{0}bar""#);
+    }
+
+    #[test]
+    fn not_quote_unicode_unnecessarily() {
+        assert_eq!(quote_and_escape("mikuličić"), "mikuličić");
+    }
 }