Rename uptime description for Prometheus in rename filter

Prometheus prescribes what the description of the uptime metric should be[1], and while it does not enforce this on the server, the pushgateway complains.

[1] https://prometheus.io/docs/instrumenting/writing_clientlibs/#process-metrics

Closes micrometer-metricsgh-5290

implementations/micrometer-registry-prometheus/src/main/java/io/micrometer/prometheusmetrics/PrometheusRenameFilter.java

@@ -16,6 +16,7 @@
 package io.micrometer.prometheusmetrics;
 
 import io.micrometer.core.instrument.Meter;
+import io.micrometer.core.instrument.Tags;
 import io.micrometer.core.instrument.config.MeterFilter;
 
 import java.util.HashMap;
@@ -39,6 +40,10 @@ public class PrometheusRenameFilter implements MeterFilter {
 
     @Override
     public Meter.Id map(Meter.Id id) {
+        if (id.getName().equals("process.start.time")) {
+            return new Meter.Id(id.getName(), Tags.of(id.getTagsAsIterable()), id.getBaseUnit(),
+                    "Start time of the process since unix epoch in seconds.", id.getType());
+        }
         String convertedName = MICROMETER_TO_PROMETHEUS_NAMES.get(id.getName());
         return convertedName == null ? id : id.withName(convertedName);
     }

implementations/micrometer-registry-prometheus/src/test/java/io/micrometer/prometheusmetrics/PrometheusRenameFilterTest.java

@@ -15,9 +15,11 @@
  */
 package io.micrometer.prometheusmetrics;
 
+import io.micrometer.core.Issue;
 import io.micrometer.core.instrument.Gauge;
 import io.micrometer.core.instrument.Meter;
 import io.micrometer.core.instrument.MeterRegistry;
+import io.micrometer.core.instrument.binder.system.UptimeMetrics;
 import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
 import org.junit.jupiter.api.Test;
 
@@ -49,4 +51,13 @@ void doesChangeApplicableMeter() {
         assertThat(actual.getName()).isEqualTo("process.open.fds");
     }
 
+    @Test
+    @Issue("#5290")
+    void processStartTimeDescriptionMatchesPrometheusHelpString() {
+        registry.config().meterFilter(filter);
+        new UptimeMetrics().bindTo(registry);
+        assertThat(registry.get("process.start.time").meter().getId().getDescription())
+            .isEqualTo("Start time of the process since unix epoch in seconds.");
+    }
+
 }