[receiver/loki] Add structured metadata support (open-telemetry#40096)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Added support for [Structured
Metadata](https://grafana.com/docs/loki/latest/get-started/labels/structured-metadata/).
If Structured Metadata is present in the loki log entry it will be
converted to Log Record Attributes

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes
open-telemetry#40095

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Added a unit test to verify the translation of Structured metadata to
log record attributes

Co-authored-by: Antoine Toulme <[email protected]>

Author: 2 people

.chloggen/loki-receiver-structured-metadata.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: lokireceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add support for structured metadata in lokireceiver
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [40095]
+
+# (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/translator/loki/loki_to_otlp.go

@@ -78,4 +78,7 @@ func ConvertEntryToLogRecord(entry *push.Entry, lr *plog.LogRecord, labelSet mod
 	for key, value := range labelSet {
 		lr.Attributes().PutStr(string(key), string(value))
 	}
+	for _, metadata := range entry.StructuredMetadata {
+		lr.Attributes().PutStr(metadata.Name, metadata.Value)
+	}
 }

pkg/translator/loki/loki_to_otlp_test.go

@@ -77,6 +77,40 @@ func TestPushRequestToLogs(t *testing.T) {
 				},
 			}),
 		},
+		{
+			name: "Should add structured metadata to log record attributes",
+			pushRequest: &push.PushRequest{
+				Streams: []push.Stream{
+					{
+						Labels: "{foo=\"bar\", label1=\"value1\"}",
+						Entries: []push.Entry{
+							{
+								Timestamp: time.Unix(0, 1676888496000000000),
+								Line:      "logline 1",
+								StructuredMetadata: push.LabelsAdapter{
+									{
+										Name:  "sm-key1",
+										Value: "sm-value1",
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+			keepTimestamp: true,
+			expected: generateLogs([]Log{
+				{
+					Timestamp: 1676888496000000000,
+					Body:      pcommon.NewValueStr("logline 1"),
+					Attributes: map[string]any{
+						"foo":     "bar",
+						"label1":  "value1",
+						"sm-key1": "sm-value1",
+					},
+				},
+			}),
+		},
 	}
 
 	for _, tt := range testCases {