restore '*' as wildcard + clarify quoting

Author: SylvainJuge

instrumentation/jmx-metrics/javaagent/README.md

@@ -239,7 +239,7 @@ rules:
           connector_state:
             ok: STARTED
             failed: [STOPPED,FAILED]
-            degraded: _
+            degraded: '*'
 ```
 
 For a given value of `port`, let's say `8080` This will capture the `tomcat.connector.state` metric of type `updowncounter` with value `0` or `1` and the `state` metric attribute will have a value in [`ok`,`failed`,`degraded`].
@@ -265,21 +265,21 @@ For other values of `stateName`, we have:
 
 Each state key can be mapped to one or more values of the MBean attribute using:
 - a string literal or a string array
-- a `_` character to provide default option and avoid enumerating all values.
+- a `*` character to provide default option and avoid enumerating all values, this value must be quoted in YAML
 
-Exactly one `_` value must be present in the mapping to ensure all possible values of the MBean attribute can be mapped to a state key.
+Exactly one `*` value must be present in the mapping to ensure all possible values of the MBean attribute can be mapped to a state key.
 
-The default value indicated by `_` does not require a dedicated state key. For example, if we want to have `connector_state` metric attribute with values `on` or `off`, we can use:
+The default value indicated by `*` does not require a dedicated state key. For example, if we want to have `connector_state` metric attribute with values `on` or `off`, we can use:
 ```yaml
           connector_state:
             on: STARTED
-            off: [STOPPED,FAILED,_]
+            off: [STOPPED,FAILED,'*']
 ```
 In the particular case where only two values are defined, we can simplify further by explicitly defining one state and rely on default for the other.
 ```yaml
           connector_state:
             on: STARTED
-            off: _
+            off: '*'
 ```
 
 ### General Syntax

instrumentation/jmx-metrics/library/src/main/java/io/opentelemetry/instrumentation/jmx/yaml/MetricStructure.java

@@ -44,7 +44,7 @@ abstract class MetricStructure {
 
   private Map<String, Object> metricAttribute;
   private StateMapping stateMapping = StateMapping.empty();
-  private static final String STATE_MAPPING_DEFAULT = "_";
+  private static final String STATE_MAPPING_DEFAULT = "*";
   private String unit;
 
   private MetricInfo.Type metricType;

instrumentation/jmx-metrics/library/src/test/java/io/opentelemetry/instrumentation/jmx/engine/RuleParserTest.java

@@ -373,7 +373,7 @@ void testConf8() throws Exception {
           + "          state_attribute: \n" // --> only one state attribute allowed
           + "            ok: STARTED\n" // as simple string
           + "            failed: [STOPPED,FAILED]\n" // as array of strings
-          + "            degraded: _\n" // degraded value for default
+          + "            degraded: '*'\n" // degraded value for default
           + "";
 
   @Test