journald: allow custom journal fields (#2708)

## Motivation

It's currently not possible to customize how messages will get send to journald.

This became apparent in #2425, where first a specific API got designed, but then
it was decided that users should not get restricted in only a subset of fields,
but should be able to simply choose by themselves what fields get set with what
values.

So in a sense, this is the successor/rework of #2425.

## Solution

Allow custom fields to be set in tracing-journald.

## Open Questions

- [x] How should we deal with fields that also get supplied by other options?
  For example, setting `SYSLOG_IDENTIFIER` here and also setting
  `.with_syslog_identifier()` will send said field twice, potentially with
  differing values. Is that a problem?
    - Answer: No, this is not a problem.

Closes #2425

tracing-journald/src/lib.rs

@@ -85,6 +85,7 @@ pub struct Subscriber {
     socket: UnixDatagram,
     field_prefix: Option<String>,
     syslog_identifier: String,
+    additional_fields: Vec<u8>,
 }
 
 #[cfg(unix)]
@@ -109,6 +110,7 @@ impl Subscriber {
                     .map(|n| n.to_string_lossy().into_owned())
                     // If we fail to get the name of the current executable fall back to an empty string.
                     .unwrap_or_else(String::new),
+                additional_fields: Vec::new(),
             };
             // Check that we can talk to journald, by sending empty payload which journald discards.
             // However if the socket didn't exist or if none listened we'd get an error here.
@@ -150,6 +152,40 @@ impl Subscriber {
         self
     }
 
+    /// Adds fields that will get be passed to journald with every log entry.
+    ///
+    /// The input values of this function are interpreted as `(field, value)` pairs.
+    ///
+    /// This can for example be used to configure the syslog facility.
+    /// See [Journal Fields](https://www.freedesktop.org/software/systemd/man/systemd.journal-fields.html)
+    /// and [journalctl](https://www.freedesktop.org/software/systemd/man/journalctl.html)
+    /// for more information.
+    ///
+    /// Fields specified using this method will be added to the journald
+    /// message alongside fields generated from the event's fields, its
+    /// metadata, and the span context. If the name of a field provided using
+    /// this method is the same as the name of a field generated by the
+    /// subscriber, both fields will be sent to journald.
+    ///
+    /// ```no_run
+    /// # use tracing_journald::Subscriber;
+    /// let sub = Subscriber::new()
+    ///     .unwrap()
+    ///     .with_custom_fields([("SYSLOG_FACILITY", "17")]);
+    /// ```
+    ///
+    pub fn with_custom_fields<T: AsRef<str>, U: AsRef<[u8]>>(
+        mut self,
+        fields: impl IntoIterator<Item = (T, U)>,
+    ) -> Self {
+        for (name, value) in fields {
+            put_field_length_encoded(&mut self.additional_fields, name.as_ref(), |buf| {
+                buf.extend_from_slice(value.as_ref())
+            })
+        }
+        self
+    }
+
     /// Returns the syslog identifier in use.
     pub fn syslog_identifier(&self) -> &str {
         &self.syslog_identifier
@@ -257,6 +293,7 @@ where
         put_field_length_encoded(&mut buf, "SYSLOG_IDENTIFIER", |buf| {
             write!(buf, "{}", self.syslog_identifier).unwrap()
         });
+        buf.extend_from_slice(&self.additional_fields);
 
         event.record(&mut EventVisitor::new(
             &mut buf,

tracing-journald/tests/journal.rs

@@ -237,6 +237,28 @@ fn simple_metadata() {
     });
 }
 
+#[test]
+fn journal_fields() {
+    let sub = Subscriber::new()
+        .unwrap()
+        .with_field_prefix(None)
+        .with_custom_fields([("SYSLOG_FACILITY", "17")])
+        .with_custom_fields([("ABC", "dEf"), ("XYZ", "123")]);
+    with_journald_subscriber(sub, || {
+        info!(test.name = "journal_fields", "Hello World");
+
+        let message = retry_read_one_line_from_journal("journal_fields");
+        assert_eq!(message["MESSAGE"], "Hello World");
+        assert_eq!(message["PRIORITY"], "5");
+        assert_eq!(message["TARGET"], "journal");
+        assert_eq!(message["SYSLOG_FACILITY"], "17");
+        assert_eq!(message["ABC"], "dEf");
+        assert_eq!(message["XYZ"], "123");
+        assert!(message["CODE_FILE"].as_text().is_some());
+        assert!(message["CODE_LINE"].as_text().is_some());
+    });
+}
+
 #[test]
 fn span_metadata() {
     with_journald(|| {