switch to _ + enhance doc for 2 state

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`].
@@ -265,9 +265,22 @@ 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 `*` wildcard to provide default option and avoid enumerating all values.
+- a `_` character to provide default option and avoid enumerating all values.
 
-Exactly one wildcard 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:
+```yaml
+          connector_state:
+            on: STARTED
+            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: _
+```
 
 ### 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_WILDCARD = "*";
+  private static final String STATE_MAPPING_DEFAULT = "_";
   private String unit;
 
   private MetricInfo.Type metricType;
@@ -68,7 +68,7 @@ public void setUnit(String unit) {
 
   private static void addMappedValue(
       StateMapping.Builder builder, String stateValue, String stateKey) {
-    if (stateValue.equals(STATE_MAPPING_WILDCARD)) {
+    if (stateValue.equals(STATE_MAPPING_DEFAULT)) {
       builder.withDefaultState(stateKey);
     } else {
       builder.withMappedValue(stateValue, stateKey);

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" // wildcard value for default
+          + "            degraded: _\n" // degraded value for default
           + "";
 
   @Test