[receiver/journald]: add support for dmesg (#25910)

Adds support for `--dmesg` to journaldreceiver

Author: sumo-drosiek

.chloggen/drosiek-journald.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: add support for dmesg
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [20295]
+
+# (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: []

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

@@ -60,6 +60,7 @@ type Config struct {
 	Priority  string        `mapstructure:"priority,omitempty"`
 	Matches   []MatchConfig `mapstructure:"matches,omitempty"`
 	Grep      string        `mapstructure:"grep,omitempty"`
+	Dmesg     bool          `mapstructure:"dmesg,omitempty"`
 }
 
 type MatchConfig map[string]string
@@ -119,6 +120,10 @@ func (c Config) buildArgs() ([]string, error) {
 		args = append(args, "--grep", c.Grep)
 	}
 
+	if c.Dmesg {
+		args = append(args, "--dmesg")
+	}
+
 	switch {
 	case c.Directory != nil:
 		args = append(args, "--directory", *c.Directory)

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

@@ -190,6 +190,13 @@ func TestBuildConfig(t *testing.T) {
 			},
 			Expected: []string{"--utc", "--output=json", "--follow", "--priority", "info", "--grep", "test_grep"},
 		},
+		{
+			Name: "dmesg",
+			Config: func(cfg *Config) {
+				cfg.Dmesg = true
+			},
+			Expected: []string{"--utc", "--output=json", "--follow", "--priority", "info", "--dmesg"},
+		},
 	}
 
 	for _, tt := range testCases {

receiver/journaldreceiver/README.md

@@ -32,6 +32,7 @@ Journald receiver requires that:
 | `matches`                           |                                      | A list of matches to read entries from. See [Matches](#matches) and [Multiple filtering options](#multiple-filtering-options) examples.                                                                                                  |
 | `priority`                          | `info`                               | Filter output by message priorities or priority ranges. See [Multiple filtering options](#multiple-filtering-options) examples.                                                                                                          |
 | `grep`                              |                                      | Filter output to entries where the MESSAGE= field matches the specified regular expression. See [Multiple filtering options](#multiple-filtering-options) examples.                                                                      |
+| `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. |
 | `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.                                                                                                                                                                                    |
@@ -85,6 +86,8 @@ which is going to retrieve all entries which match at least one of the following
 In case of using multiple following options, conditions between them are logically `AND`ed and within them are logically `OR`ed:
 
 ```text
+( dmesg )
+AND
 ( priority )
 AND
 ( units[0] OR units[1] OR units[2] OR ... units[U] )