Fix broken content negotiation for Prometheus with OpenMetrics

Update Prometheus `TextOutputFormat` so that OpenMetrics is used in
preference to text output when an appropriate accept header is found.

If the accept header contains `*/*` or is missing then the text format
will be used.

See gh-28130

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/metrics/export/prometheus/TextOutputFormat.java

@@ -36,25 +36,30 @@ import org.springframework.util.MimeTypeUtils;
 public enum TextOutputFormat implements Producible<TextOutputFormat> {
 
 	/**
-	 * OpenMetrics text version 1.0.0.
+	 * Prometheus text version 0.0.4.
 	 */
-	CONTENT_TYPE_OPENMETRICS_100(TextFormat.CONTENT_TYPE_OPENMETRICS_100) {
+	CONTENT_TYPE_004(TextFormat.CONTENT_TYPE_004) {
 
 		@Override
 		void write(Writer writer, Enumeration<MetricFamilySamples> samples) throws IOException {
-			TextFormat.writeOpenMetrics100(writer, samples);
+			TextFormat.write004(writer, samples);
+		}
+
+		@Override
+		public boolean isDefault() {
+			return true;
 		}
 
 	},
 
 	/**
-	 * Prometheus text version 0.0.4.
+	 * OpenMetrics text version 1.0.0.
 	 */
-	CONTENT_TYPE_004(TextFormat.CONTENT_TYPE_004) {
+	CONTENT_TYPE_OPENMETRICS_100(TextFormat.CONTENT_TYPE_OPENMETRICS_100) {
 
 		@Override
 		void write(Writer writer, Enumeration<MetricFamilySamples> samples) throws IOException {
-			TextFormat.write004(writer, samples);
+			TextFormat.writeOpenMetrics100(writer, samples);
 		}
 
 	};

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/export/prometheus/PrometheusScrapeEndpointIntegrationTests.java

@@ -55,6 +55,14 @@ class PrometheusScrapeEndpointIntegrationTests {
 						.contains("counter1_total").contains("counter2_total").contains("counter3_total"));
 	}
 
+	@WebEndpointTest
+	void scrapePrefersToProduceOpenMetrics100(WebTestClient client) {
+		MediaType openMetrics = MediaType.parseMediaType(TextFormat.CONTENT_TYPE_OPENMETRICS_100);
+		MediaType textPlain = MediaType.parseMediaType(TextFormat.CONTENT_TYPE_004);
+		client.get().uri("/actuator/prometheus").accept(openMetrics, textPlain).exchange().expectStatus().isOk()
+				.expectHeader().contentType(openMetrics);
+	}
+
 	@WebEndpointTest
 	void scrapeWithIncludedNames(WebTestClient client) {
 		client.get().uri("/actuator/prometheus?includedNames=counter1_total,counter2_total").exchange().expectStatus()