tmp commit: manually tested all previous OpenTelemetry versions

README.md

@@ -298,6 +298,8 @@ class YourClass {
 
 ## Exemplars
 
+TODO: Document that Exemplars are supported with OpenTelemetry 0.16.0 or higher. When `client_java` 0.11.0 was released the current OpenTelemetry version was 1.1.0.
+
 Exemplars are a feature of the [OpenMetrics](http://openmetrics.io) format that allows applications to link metrics
 to example traces. Starting with version 0.11.0, `client_java` automatically adds Exemplars for applications that
 are instrumented with [OpenTelemetry](https://opentelemetry.io/) distributed tracing. No code change is required.

integration_tests/exemplars_otel/src/main/java/io/prometheus/it/exemplars_otel/Server.java

@@ -17,11 +17,57 @@ public static void main(String[] args) throws IOException, InterruptedException
         .help("help")
         .register();
 
+    // The following code works with OpenTelemetry versions 0.13.0 and higher.
+    // Exemplars work with 0.16.0 and higher. With 0.15.0 or lower the example will run but not have exemplars.
+    System.out.println(Tracer.class.getProtectionDomain().getCodeSource().getLocation());
     Tracer tracer = SdkTracerProvider.builder().build().get(null);
     Span span = tracer.spanBuilder("my span").startSpan();
     span.makeCurrent();
     counter.inc(1);
     span.end();
     Thread.currentThread().join(); // sleep forever
+
+    // Examples with older OpenTelemetry versions used for manual testing:
+
+    /*
+    // OpenTelemetry versions 0.10.0 - 0.12.0
+    System.out.println(Tracer.class.getProtectionDomain().getCodeSource().getLocation());
+    Tracer tracer = OpenTelemetrySdk.get().getTracer("test");
+    Span span = tracer.spanBuilder("my span").startSpan();
+    span.makeCurrent();
+    counter.inc(1);
+    span.end();
+    Thread.currentThread().join(); // sleep forever
+     */
+
+    /*
+    // OpenTelemetry versions 0.4.0 - 0.9.1
+    System.out.println(TracerSdkProvider.class.getProtectionDomain().getCodeSource().getLocation());
+    Tracer tracer = TracerSdkProvider.builder().build().get("test");
+    Span span = tracer.spanBuilder("my span").startSpan();
+    counter.inc(1);
+    span.end();
+    Thread.currentThread().join(); // sleep forever
+     */
+
+    /*
+    // OpenTelemetry version 0.3.0
+    System.out.println(TracerSdkProvider.class.getProtectionDomain().getCodeSource().getLocation());
+    TracerSdk tracer = TracerSdkProvider.builder().build().get("test");
+    Span span = tracer.spanBuilder("my span").startSpan();
+    counter.inc(1);
+    span.end();
+    Thread.currentThread().join(); // sleep forever
+     */
+
+    /*
+    // OpenTelemetry version 0.2.0
+    System.out.println(TracerSdkFactory.class.getProtectionDomain().getCodeSource().getLocation());
+    TracerSdk tracer = TracerSdkFactory.create().get("test");
+    Span span = tracer.spanBuilder("my span").startSpan();
+    counter.inc(1);
+    span.end();
+    Thread.currentThread().join(); // sleep forever
+     */
   }
 }

tracer_parent/tracer_otel/src/main/java/io/prometheus/client/exemplars/tracer/otel/OpenTelemetrySpanContextSupplier.java

@@ -13,7 +13,11 @@ public static boolean isAvailable() {
       test.getSpanId();
       test.getSpanId();
       return true;
-    } catch (NoClassDefFoundError e) {
+    } catch (LinkageError ignored) {
+      // NoClassDefFoundError:
+      //   Either OpenTelemetry is not present, or it is version 0.9.1 or older when io.opentelemetry.api.trace.Span did not exist.
+      // IncompatibleClassChangeError:
+      //   The application uses an OpenTelemetry version between 0.10.0 and 0.15.0 when SpanContext was a class, and not an interface.
       return false;
     }
   }

tracer_parent/tracer_otel_agent/src/main/java/io/prometheus/client/exemplars/tracer/otel_agent/OpenTelemetryAgentSpanContextSupplier.java

@@ -18,7 +18,11 @@ public static boolean isAvailable() {
       test.getSpanId();
       test.getSpanId();
       return true;
-    } catch (NoClassDefFoundError e) {
+    } catch (LinkageError ignored) {
+      // NoClassDefFoundError:
+      //   Either OpenTelemetry is not present, or it is version 0.9.1 or older when io.opentelemetry.api.trace.Span did not exist.
+      // IncompatibleClassChangeError:
+      //   The application uses an OpenTelemetry version between 0.10.0 and 0.15.0 when SpanContext was a class, and not an interface.
       return false;
     }
   }