Don't sanitize away colons in metric names (#498)

The docs specify that colons (`:`) are allowed in metric names, but
`Collector.sanitizeMetricName` removes them.

Signed-off-by: ugniusb <ugniusb@wix.com>

Author: ugniusb

simpleclient/src/main/java/io/prometheus/client/Collector.java

@@ -183,8 +183,8 @@ protected static void checkMetricName(String name) {
     }
   }
 
-  private static final Pattern SANITIZE_PREFIX_PATTERN = Pattern.compile("^[^a-zA-Z_]");
-  private static final Pattern SANITIZE_BODY_PATTERN = Pattern.compile("[^a-zA-Z0-9_]");
+  private static final Pattern SANITIZE_PREFIX_PATTERN = Pattern.compile("^[^a-zA-Z_:]");
+  private static final Pattern SANITIZE_BODY_PATTERN = Pattern.compile("[^a-zA-Z0-9_:]");
 
   /**
    * Sanitize metric name

simpleclient/src/test/java/io/prometheus/client/CollectorTest.java

@@ -9,5 +9,6 @@ public class CollectorTest {
   public void sanitizeMetricName() throws Exception {
       assertEquals("_hoge", Collector.sanitizeMetricName("0hoge"));
       assertEquals("foo_bar0", Collector.sanitizeMetricName("foo.bar0"));
+      assertEquals(":baz::", Collector.sanitizeMetricName(":baz::"));
   }
 }