[receiver/journald] adds namespace support (#36031)

#### Description

Adds support for querying journald namespaces
See
https://www.freedesktop.org/software/systemd/man/latest/systemd-journald.service.html#Journal%20Namespaces
for details

#### Testing

Adds tests for correctly generating the `journalctl` command.

Tested on a real world example as well with the resulting binary

#### Documentation

Adds description of the setting the PR introduces

.chloggen/36031-add-namespace-support.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: 'enhancement'
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: journaldreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: allows querying a journald namespace
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [36031]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

pkg/stanza/operator/input/journald/config_all.go

@@ -37,6 +37,7 @@ type Config struct {
 	Grep        string        `mapstructure:"grep,omitempty"`
 	Dmesg       bool          `mapstructure:"dmesg,omitempty"`
 	All         bool          `mapstructure:"all,omitempty"`
+	Namespace   string        `mapstructure:"namespace,omitempty"`
 }
 
 type MatchConfig map[string]string

pkg/stanza/operator/input/journald/config_linux.go

@@ -90,6 +90,10 @@ func (c Config) buildArgs() ([]string, error) {
 		args = append(args, "--dmesg")
 	}
 
+	if len(c.Namespace) > 0 {
+		args = append(args, "--namespace", c.Namespace)
+	}
+
 	switch {
 	case c.Directory != nil:
 		args = append(args, "--directory", *c.Directory)

pkg/stanza/operator/input/journald/input_test.go

@@ -200,6 +200,13 @@ func TestBuildConfig(t *testing.T) {
 			},
 			Expected: []string{"--utc", "--output=json", "--follow", "--priority", "info", "--grep", "test_grep"},
 		},
+		{
+			Name: "namespace",
+			Config: func(cfg *Config) {
+				cfg.Namespace = "foo"
+			},
+			Expected: []string{"--utc", "--output=json", "--follow", "--priority", "info", "--namespace", "foo"},
+		},
 		{
 			Name: "dmesg",
 			Config: func(cfg *Config) {

receiver/journaldreceiver/README.md

@@ -35,6 +35,7 @@ Journald receiver requires that:
 | `dmesg`                             | 'false'                              | Show only kernel messages. This shows logs from current boot and adds the match `_TRANSPORT=kernel`. See [Multiple filtering options](#multiple-filtering-options) examples.                                                             |
 | `storage`                           | none                                 | The ID of a storage extension to be used to store cursors. Cursors allow the receiver to pick up where it left off in the case of a collector restart. If no storage extension is used, the receiver will manage cursors in memory only. |
 | `all`                               | 'false'                              | If `true`, very long logs and logs with unprintable characters will also be included.                                                                                                                                                    |
+| `namespace`                         |                                      | Will query the given namespace. See man page [`systemd-journald.service(8)`](https://www.man7.org/linux/man-pages/man8/systemd-journald.service.8.html#JOURNAL_NAMESPACES) for details.                                                  |
 | `retry_on_failure.enabled`          | `false`                              | If `true`, the receiver will pause reading a file and attempt to resend the current batch of logs if it encounters an error from downstream components.                                                                                  |
 | `retry_on_failure.initial_interval` | `1 second`                           | Time to wait after the first failure before retrying.                                                                                                                                                                                    |
 | `retry_on_failure.max_interval`     | `30 seconds`                         | Upper bound on retry backoff interval. Once this value is reached the delay between consecutive retries will remain constant at the specified value.                                                                                     |