fix(native-spans): let an explicit agent exporter win in a Lambda

The Lambda no-local-agent probe selected the stdout exporter whenever
neither the extension layer nor the mini agent was present, without
looking at what the user had configured. `getExporter` on master matched
the configured name in a switch and returned before it ever reached that
probe, so `exporter: 'agent'` in a Lambda kept using the agent.

Restoring that precedence fixes suites that set both
`DD_TRACE_EXPERIMENTAL_EXPORTER=agent` and `AWS_LAMBDA_FUNCTION_NAME`
and then wait for traces at the test agent: their spans were being
written to stdout instead, so every assertion timed out. The fetch
plugin's "in serverless" block went from four 25s timeouts to passing,
and the whole suite from 4m to 2s.

Author: bengl

packages/dd-trace/src/opentracing/tracer.js

@@ -92,9 +92,15 @@ class DatadogTracer {
     // instead. Probe for both markers exactly as the pre-native-spans exporter
     // selection did, otherwise these functions POST every span to a loopback port
     // nothing listens on (config forces flushInterval=0 there) and lose all traces.
+    //
+    // An explicit `exporter: 'agent'` still wins: master's `getExporter` matched
+    // the configured name in a switch and returned before it ever reached this
+    // probe, so a Lambda told to use the agent must use the agent.
+    //
     // Kept independent of `useLambdaJsPipeline` (which excludes OTLP) so the
     // missing-libdatadog degrade path below can reuse it.
     const lambdaWithoutLocalAgent = getIsAWSLambda() &&
+      configuredExporter !== exporters.AGENT &&
       !fs.existsSync(DATADOG_LAMBDA_EXTENSION_PATH) &&
       !fs.existsSync(DATADOG_MINI_AGENT_PATH)
     const useLambdaLogExporter = useLambdaJsPipeline && lambdaWithoutLocalAgent