Nomralize results of keys(), log a one time warning

Author: jack-berg

api/incubator/src/main/java/io/opentelemetry/api/incubator/propagation/EnvironmentGetter.java

@@ -10,6 +10,9 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.logging.Level;
+import java.util.logging.Logger;
 import javax.annotation.Nullable;
 
 /**
@@ -43,6 +46,8 @@
  */
 public final class EnvironmentGetter implements TextMapGetter<Map<String, String>> {
 
+  private static final AtomicBoolean LOG_KEYS_CALLED = new AtomicBoolean(false);
+  private static final Logger LOGGER = Logger.getLogger(EnvironmentGetter.class.getName());
   private static final EnvironmentGetter INSTANCE = new EnvironmentGetter();
 
   private EnvironmentGetter() {}
@@ -54,6 +59,13 @@ public static EnvironmentGetter getInstance() {
 
   @Override
   public Iterable<String> keys(Map<String, String> carrier) {
+    if (LOG_KEYS_CALLED.compareAndSet(false, true)) {
+      LOGGER.log(
+          Level.WARNING,
+          "keys() called on EnvironmentGetter. "
+              + "This may produce unexpected results with propagators which depend on case sensitivity or special characters in keys.",
+          new Throwable());
+    }
     if (carrier == null) {
       return Collections.emptyList();
     }

api/incubator/src/test/java/io/opentelemetry/api/incubator/propagation/EnvironmentGetterTest.java

@@ -7,13 +7,19 @@
 
 import static io.opentelemetry.sdk.testing.assertj.OpenTelemetryAssertions.assertThat;
 
+import io.github.netmikey.logunit.api.LogCapturer;
+import io.opentelemetry.internal.testing.slf4j.SuppressLogger;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
 
 class EnvironmentGetterTest {
 
+  @RegisterExtension
+  LogCapturer logCapturer = LogCapturer.create().captureForType(EnvironmentGetter.class);
+
   @Test
   void get() {
     Map<String, String> carrier = new HashMap<>();
@@ -45,6 +51,7 @@ void get_null() {
   }
 
   @Test
+  @SuppressLogger(EnvironmentGetter.class)
   void keys_valuesAreNormalized() {
     Map<String, String> carrier = new HashMap<>();
     carrier.put("otel.trace.id", "V1");
@@ -53,6 +60,9 @@ void keys_valuesAreNormalized() {
     assertThat(EnvironmentGetter.getInstance().keys(carrier))
         .containsExactlyInAnyOrder("OTEL_TRACE_ID", "OTEL_BAGGAGE_KEY");
     assertThat(EnvironmentGetter.getInstance().keys(null)).isEmpty();
+
+    assertThat(logCapturer.size()).isEqualTo(1);
+    logCapturer.assertContains("keys() called on EnvironmentGetter");
   }
 
   @Test