Add second environment variable to configure ikey

None of the other SDKs use the `APPLICATION_INSIGHTS_IKEY` environment
variable to configure the instrumentation key for Application Insights
and instead use `APPINSIGHTS_INSTRUMENTATIONKEY`.

For consistency with the other SDKs (e.g. Dot-Net https://aka.ms/n2ruy1
or Node https://aka.ms/v8r0pk) this Java SDK should also support the
more common environment variable name to avoid confusion for developers
who have experience with the non-Java SDKs.

Author: c-w

core/src/main/java/com/microsoft/applicationinsights/internal/config/TelemetryConfigurationFactory.java

@@ -60,6 +60,7 @@ public enum TelemetryConfigurationFactory {
     private String performanceCountersSection = DEFAULT_PERFORMANCE_MODULES_PACKAGE;
 
     final static String EXTERNAL_PROPERTY_IKEY_NAME = "APPLICATION_INSIGHTS_IKEY";
+    final static String EXTERNAL_PROPERTY_IKEY_NAME_SECONDARY = "APPINSIGHTS_INSTRUMENTATIONKEY";
 
     private AppInsightsConfigurationBuilder builder = new JaxbAppInsightsConfigurationBuilder();
 
@@ -208,28 +209,39 @@ private void setTelemetryProcessors(ApplicationInsightsXmlConfiguration appConfi
 
     /**
      * Setting an instrumentation key:
-     * First we try the system property '-DAPPLICATION_INSIGHTS_IKEY=i_key'
-     * Next we will try the environment variable 'APPLICATION_INSIGHTS_IKEY',
+     * First we try the system property '-DAPPLICATION_INSIGHTS_IKEY=i_key' or '-DAPPINSIGHTS_INSTRUMENTATIONKEY=i_key'
+     * Next we will try the environment variable 'APPLICATION_INSIGHTS_IKEY' or 'APPINSIGHTS_INSTRUMENTATIONKEY'
      * Next we will try to fetch the i-key from the ApplicationInsights.xml
      * @param userConfiguration The configuration that was represents the user's configuration in ApplicationInsights.xml.
      * @param configuration The configuration class.
      */
     private void setInstrumentationKey(ApplicationInsightsXmlConfiguration userConfiguration, TelemetryConfiguration configuration) {
         try {
-            // First, check whether an i-key was provided as a java system property i.e. '-DAPPLICATION_INSIGHTS_IKEY=i_key'
-            String ikey = System.getProperty(EXTERNAL_PROPERTY_IKEY_NAME);
+            String ikey;
 
+            // First, check whether an i-key was provided as a java system property i.e. '-DAPPLICATION_INSIGHTS_IKEY=i_key', or '-DAPPINSIGHTS_INSTRUMENTATIONKEY=i_key'
+            ikey = System.getProperty(EXTERNAL_PROPERTY_IKEY_NAME);
+            if (!Strings.isNullOrEmpty(ikey)) {
+                configuration.setInstrumentationKey(ikey);
+                return;
+            }
+            ikey = System.getProperty(EXTERNAL_PROPERTY_IKEY_NAME_SECONDARY);
             if (!Strings.isNullOrEmpty(ikey)) {
                 configuration.setInstrumentationKey(ikey);
                 return;
             }
 
-            // Second, try to find the i-key as an environment variable 'APPLICATION_INSIGHTS_IKEY'
+            // Second, try to find the i-key as an environment variable 'APPLICATION_INSIGHTS_IKEY' or 'APPINSIGHTS_INSTRUMENTATIONKEY'
             ikey = System.getenv(EXTERNAL_PROPERTY_IKEY_NAME);
             if (!Strings.isNullOrEmpty(ikey)) {
                 configuration.setInstrumentationKey(ikey);
                 return;
             }
+            ikey = System.getenv(EXTERNAL_PROPERTY_IKEY_NAME_SECONDARY);
+            if (!Strings.isNullOrEmpty(ikey)) {
+                configuration.setInstrumentationKey(ikey);
+                return;
+            }
 
             // Else, try to find the i-key in ApplicationInsights.xml
             if (userConfiguration != null) {

core/src/test/java/com/microsoft/applicationinsights/internal/config/TelemetryConfigurationFactoryTest.java

@@ -117,6 +117,17 @@ public void systemPropertyIKeyBeforeConfigurationIKeyTest() {
         }
     }
 
+    @Test
+    public void systemPropertyIKeySecondaryBeforeConfigurationIKeyTest() {
+        try {
+            System.setProperty(TelemetryConfigurationFactory.EXTERNAL_PROPERTY_IKEY_NAME_SECONDARY, APP_INSIGHTS_IKEY_TEST_VALUE);
+            ikeyTest(MOCK_IKEY, APP_INSIGHTS_IKEY_TEST_VALUE);
+        } finally {
+            // Avoid any influence on other unit tests
+            System.getProperties().remove(TelemetryConfigurationFactory.EXTERNAL_PROPERTY_IKEY_NAME_SECONDARY);
+        }
+    }
+
     @Test
     public void testWithEmptySections() {
         AppInsightsConfigurationBuilder mockParser = Mockito.mock(AppInsightsConfigurationBuilder.class);