Tweak performance for Prometheus scraping endpoint

Reduce the number of times capacity growth is needed inside the StringWriter.
A typical default SpringBoot Prometheus page has more than 11k characters.
Best performance results when no capacity growth is needed at all, so base
it on previous metrics page size plus some room for possible extra metric info.

See gh-30085

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

@@ -42,21 +42,29 @@
 @WebEndpoint(id = "prometheus")
 public class PrometheusScrapeEndpoint {
 
+	private static final int METRICS_SCRAPE_CHARS_EXTRA = 1024;
+
 	private final CollectorRegistry collectorRegistry;
 
+	private volatile int nextMetricsScrapeSize = 16;
+
 	public PrometheusScrapeEndpoint(CollectorRegistry collectorRegistry) {
 		this.collectorRegistry = collectorRegistry;
 	}
 
 	@ReadOperation(producesFrom = TextOutputFormat.class)
 	public WebEndpointResponse<String> scrape(TextOutputFormat format, @Nullable Set<String> includedNames) {
 		try {
-			Writer writer = new StringWriter();
+			Writer writer = new StringWriter(nextMetricsScrapeSize);
 			Enumeration<MetricFamilySamples> samples = (includedNames != null)
 					? this.collectorRegistry.filteredMetricFamilySamples(includedNames)
 					: this.collectorRegistry.metricFamilySamples();
 			format.write(writer, samples);
-			return new WebEndpointResponse<>(writer.toString(), format);
+
+			String scrapePage = writer.toString();
+			nextMetricsScrapeSize = scrapePage.length() + METRICS_SCRAPE_CHARS_EXTRA;
+
+			return new WebEndpointResponse<>(scrapePage, format);
 		}
 		catch (IOException ex) {
 			// This actually never happens since StringWriter doesn't throw an IOException