Move exposition content type to its own var

Remove anti-pattern of setting properties on Function object that is
difficult to document.

packages/controller/src/routes.ts

@@ -95,7 +95,7 @@ async function getMetrics(req: Request, res: Response, next: any) {
 	// Should wrap results into a Generator functions ?
 	// @ts-ignore
 	let text = await lib.exposition(results);
-	res.set("Content-Type", lib.exposition.contentType);
+	res.set("Content-Type", lib.expositionContentType);
 	res.send(text);
 }
 

packages/lib/src/prometheus.ts

@@ -1664,9 +1664,6 @@ async function* expositionLines(resultsIterator: AsyncIterable<CollectorResult>)
  *     exposition for.  Defaults to collecting results from {@link
  *     defaultRegistry}.
  * @returns Prometheus exposition.
- *
- * @property exposition.contentType
- * HTTP Content-Type for the exposition format that's implemented
  */
 export async function exposition(resultsIterator = defaultRegistry.collect()) {
 	let lines = "";
@@ -1675,7 +1672,11 @@ export async function exposition(resultsIterator = defaultRegistry.collect()) {
 	}
 	return lines;
 }
-exposition.contentType = "text/plain; version=0.0.4";
+/**
+ * HTTP Content-Type for the exposition format that's implemented by {@link
+ * exposition}.
+ */
+export const expositionContentType = "text/plain; version=0.0.4";
 
 /**
  * Serialize CollectorResult into a plain object