Fix test to follow the new naming behavior of the Prometheus Client

The Prometheus Java Client used to let users to use colons (":")
in the metric names. Apparently, this should have not been allowed.

See the docs: https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels
"Metric names may contain ASCII letters, digits, underscores,
and colons. It must match the regex [a-zA-Z_:][a-zA-Z0-9_:]*.
Note: The colons are reserved for user defined recording rules.
They should not be used by exporters or direct instrumentation."

So even if metric names can contain colons (":"),
exporters (like the Prometheus Java Client or Micrometer)
should not use it. The Prometheus Java Client replaces colons (":")
to underscores ("_") starting from 1.3.3.

See prometheus/client_java#974
See prometheus/client_java#975
See #5649

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

@@ -30,7 +30,7 @@ class PrometheusNamingConventionTest {
 
     @Test
     void formatName() {
-        assertThat(convention.name("123abc/{:id}水", Meter.Type.GAUGE)).startsWith("_23abc__:id__");
+        assertThat(convention.name("123abc/{:id}水", Meter.Type.GAUGE)).startsWith("_23abc___id__");
     }
 
     @Test