fix(otel): only warn for a customer-set OTel flag missing the api peer

The guard that disables DD_LOGS_OTEL_ENABLED and DD_METRICS_OTEL_ENABLED when
@opentelemetry/api is absent warned unconditionally. If a later release defaults
one of these flags on, every app without the optional peer would get a warning
for a feature it never opted into. Warn only when the flag's origin is the
customer (tracked in trackedConfigOrigins); disable a default-on flag silently.

The config test harness gains a defaultsOverride hook so the default-on path runs
through the real #applyDefaults (which sets defaults untracked) instead of poking
internal state.

Refs: #9077 (comment)

Author: BridgeAR

packages/dd-trace/src/config/index.js

@@ -344,15 +344,17 @@ class Config extends ConfigBase {
     }
     // Both OTel pipelines require the optional @opentelemetry/api peer dep. Disable them before
     // the log-injection mutual exclusion below, so a missing module leaves DD log injection on.
+    // Warn only when the customer turned the flag on; a default-on flag is disabled silently so
+    // changing a default later does not warn every app that never opted in.
     if ((this.DD_LOGS_OTEL_ENABLED || this.DD_METRICS_OTEL_ENABLED) &&
         !require('../opentelemetry/api').isAvailable()) {
-      if (this.DD_LOGS_OTEL_ENABLED) {
-        log.warn('@opentelemetry/api is not installed; disabling DD_LOGS_OTEL_ENABLED')
-        setAndTrack(this, 'DD_LOGS_OTEL_ENABLED', false)
-      }
-      if (this.DD_METRICS_OTEL_ENABLED) {
-        log.warn('@opentelemetry/api is not installed; disabling DD_METRICS_OTEL_ENABLED')
-        setAndTrack(this, 'DD_METRICS_OTEL_ENABLED', false)
+      for (const name of ['DD_LOGS_OTEL_ENABLED', 'DD_METRICS_OTEL_ENABLED']) {
+        if (this[name]) {
+          if (trackedConfigOrigins.has(name)) {
+            log.warn('@opentelemetry/api is not installed; disabling %s', name)
+          }
+          setAndTrack(this, name, false)
+        }
       }
     }
     // Disable log injection when OTEL logs are enabled

packages/dd-trace/test/config/index.spec.js

@@ -58,6 +58,10 @@ describe('Config', () => {
     const {
       ddMajor = DD_MAJOR,
       otelApi = require('../../src/opentelemetry/api'),
+      // Merged into the resolved defaults to exercise a default-on flag (the "if we
+      // change a default later" path) through the real #applyDefaults, which sets
+      // defaults untracked.
+      defaultsOverride,
     } = overrides
 
     log = proxyquire('../../src/log', {})
@@ -72,6 +76,7 @@ describe('Config', () => {
       './parsers': parsers,
       '../../../../version': { DD_MAJOR: ddMajor },
     })
+    if (defaultsOverride) Object.assign(configDefaults.defaults, defaultsOverride)
     const configHelper = proxyquire.noPreserveCache()('../../src/config/helper', {
       './supported-configurations.json': supportedConfigurations,
     })
@@ -411,6 +416,32 @@ describe('Config', () => {
     assert.strictEqual(config.DD_LOGS_OTEL_ENABLED, true)
   })
 
+  it('silently disables a default-on OTel flag when @opentelemetry/api is not installed', () => {
+    const config = getConfig({}, {
+      otelApi: { isAvailable: () => false },
+      defaultsOverride: { DD_METRICS_OTEL_ENABLED: true, DD_LOGS_OTEL_ENABLED: true },
+    })
+
+    assert.strictEqual(config.DD_METRICS_OTEL_ENABLED, false)
+    assert.strictEqual(config.DD_LOGS_OTEL_ENABLED, false)
+    assert.ok(log.warn.getCalls().every((call) => !/@opentelemetry\/api is not installed/.test(call.args[0])))
+  })
+
+  it('warns only for the customer-set flag when a sibling is default-on', () => {
+    process.env.DD_LOGS_OTEL_ENABLED = 'true'
+
+    const config = getConfig({}, {
+      otelApi: { isAvailable: () => false },
+      defaultsOverride: { DD_METRICS_OTEL_ENABLED: true },
+    })
+
+    assert.strictEqual(config.DD_LOGS_OTEL_ENABLED, false)
+    assert.strictEqual(config.DD_METRICS_OTEL_ENABLED, false)
+    const warnings = log.warn.getCalls().filter((call) => /@opentelemetry\/api is not installed/.test(call.args[0]))
+    assert.strictEqual(warnings.length, 1)
+    assert.ok(warnings[0].args.includes('DD_LOGS_OTEL_ENABLED'))
+  })
+
   it('should initialize with OTEL environment variables when DD env vars are not set', () => {
     process.env.OTEL_SERVICE_NAME = 'otel_service'
     process.env.OTEL_LOG_LEVEL = 'debug'