fix(observability): issues with event_cache PR (vectordotdev#17768)

Ref vectordotdev#17580
Ref vectordotdev#17581

Re this PR vectordotdev#17549

This fixes a couple of issues with the mentioned PR.

- When telemetry was turned off it was still emitting the empty tags.
- The service tag was being emitted by converting the value using
`to_string`. This meant the value was delimited by `"`.
- A `telemetry` section is added to the global configuration docs.

---------

Signed-off-by: Stephen Wakely <fungus.humungus@gmail.com>

lib/vector-common/src/request_metadata.rs

@@ -25,6 +25,14 @@ impl EventCountTags {
             service: OptionalTag::Specified(None),
         }
     }
+
+    #[must_use]
+    pub fn new_unspecified() -> Self {
+        Self {
+            source: OptionalTag::Ignored,
+            service: OptionalTag::Ignored,
+        }
+    }
 }
 
 /// Must be implemented by events to get the tags that will be attached to
@@ -133,7 +141,7 @@ impl GroupedCountByteSize {
                 }
             }
             GroupedCountByteSize::Untagged { size } => {
-                event_cache.emit(&EventCountTags::new_empty(), *size);
+                event_cache.emit(&EventCountTags::new_unspecified(), *size);
             }
         }
     }

lib/vector-core/src/event/log_event.rs

@@ -224,7 +224,7 @@ impl GetEventCountTags for LogEvent {
 
         let service = if telemetry().tags().emit_service {
             self.get_by_meaning("service")
-                .map(ToString::to_string)
+                .map(|value| value.to_string_lossy().to_string())
                 .into()
         } else {
             OptionalTag::Ignored

website/cue/reference/configuration.cue

@@ -252,6 +252,62 @@ configuration: {
 			}
 		}
 
+		telemetry: {
+			common: false
+			description: """
+				Configures options for how Vector emits telemetry.
+				"""
+			required: false
+			type: object: {
+				examples: []
+				options: {
+					tags: {
+						required: false
+						description: """
+							Controls which tags should be included with the `vector_component_sent_events_total` and
+							`vector_component_sent_event_bytes_total` metrics.
+						"""
+						type: object: {
+							examples: []
+							options: {
+								emit_source: {
+									common: true
+									description: """
+										Add a `source` tag with the source component the event was received from.
+										
+										If there is no source component, for example if the event was generated by
+										the `lua` transform a `-` is emitted for this tag.
+										"""
+									required: false
+									type: bool: {
+										default: false
+									}
+								}
+								emit_service: {
+									common: false
+									description: """
+										Adds a `service` tag with the service component the event was received from.
+
+										For logs this is the field that has been determined to mean `service`. Each source may
+										define different fields for this. For example, with `syslog` events the `appname` field
+										is used.
+
+										Metric events will use the tag named `service`.
+
+										If no service is available a `-` is emitted for this tag.
+										"""
+									required: false
+									type: bool: {
+										default: false
+									}
+								}
+							}
+						}
+					}
+				}
+			}
+		}
+
 		log_schema: {
 			common: false
 			description: """