Do not set explicit unit on Prometheus Metadata

We should not set Unit in the Metadata since if it is set, it must be a
suffix of the MetricFamily name separated by an underscore.
See OpenMetrics spec:
https://github.com/OpenObservability/OpenMetrics/blob/1386544931307dff279688f332890c31b6c5de36/specification/OpenMetrics.md#unit

This can be problematic since Micrometer adds the unit to the name
but not necessarily as a suffix. For example, Micrometer does this:
test_seconds_max
but if Unit is present the name should be
test_max_seconds

The difference is not having the invalid unit definition on the output:
# UNIT test_seconds_max seconds
which would cause scraping errors on the server side
(the Prometheus 0.x registry also does this).

Closes micrometer-metricsgh-5038

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

@@ -556,8 +556,9 @@ private MetricMetadata getMetadata(Meter.Id id) {
     private MetricMetadata getMetadata(Meter.Id id, String suffix) {
         String name = config().namingConvention().name(id.getName(), id.getType(), id.getBaseUnit()) + suffix;
         String help = prometheusConfig.descriptions() ? Optional.ofNullable(id.getDescription()).orElse(" ") : " ";
-        Unit unit = id.getBaseUnit() != null ? new Unit(id.getBaseUnit()) : null;
-        return new MetricMetadata(name, help, unit);
+        // Unit is intentionally not set, see:
+        // https://github.com/OpenObservability/OpenMetrics/blob/1386544931307dff279688f332890c31b6c5de36/specification/OpenMetrics.md#unit
+        return new MetricMetadata(name, help, null);
     }
 
     private void applyToCollector(Meter.Id id, Consumer<MicrometerCollector> consumer) {

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

@@ -844,13 +844,14 @@ void meterTriggeringAnotherMeterWhenCollectingValue() {
     void openMetricsScrape() {
         Counter.builder("my.counter").baseUnit("bytes").register(registry);
         Timer.builder("my.timer").register(registry);
-        assertThat(registry.scrape("application/openmetrics-text; version=1.0.0; charset=utf-8"))
-            .contains("# TYPE my_counter_bytes counter\n" + "# UNIT my_counter_bytes bytes\n"
-                    + "# HELP my_counter_bytes  \n" + "my_counter_bytes_total 0.0\n")
-            .contains("# TYPE my_timer_seconds_max gauge\n" + "# UNIT my_timer_seconds_max seconds\n"
-                    + "# HELP my_timer_seconds_max  \n" + "my_timer_seconds_max 0.0\n")
-            .contains("# TYPE my_timer_seconds summary\n" + "# UNIT my_timer_seconds seconds\n"
-                    + "# HELP my_timer_seconds  \n" + "my_timer_seconds_count 0\n" + "my_timer_seconds_sum 0.0\n")
+        String result = registry.scrape("application/openmetrics-text; version=1.0.0; charset=utf-8");
+        assertThat(result).doesNotContain("# UNIT")
+            .contains("# TYPE my_counter_bytes counter\n" + "# HELP my_counter_bytes  \n"
+                    + "my_counter_bytes_total 0.0\n")
+            .contains("# TYPE my_timer_seconds_max gauge\n" + "# HELP my_timer_seconds_max  \n"
+                    + "my_timer_seconds_max 0.0\n")
+            .contains("# TYPE my_timer_seconds summary\n" + "# HELP my_timer_seconds  \n" + "my_timer_seconds_count 0\n"
+                    + "my_timer_seconds_sum 0.0\n")
             .endsWith("# EOF\n");
     }