docs: require AWS_LAMBDA_EXEC_WRAPPER for manual instrumentation

The Manual Instrumentation steps did not mention AWS_LAMBDA_EXEC_WRAPPER=/opt/wrapper, which is required to enable tracing (it routes the Runtime API through the extension so spans can be correlated to the current invocation). Without it, traces are discarded with "no invocation IDs". Add it as an explicit step.

Refs: SUP-1255
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: parker-edwards

README.md

@@ -125,9 +125,10 @@ The following environment variables allow fine-grained control over secret maski
 If you prefer to set up OpenTelemetry instrumentation yourself instead of relying on the extension's auto-instrumentation, you can use the manual layer and point your OTLP exporters to the extension's local endpoint. The extension will receive the telemetry, enrich it, and forward it to Dash0.
 
 1. Add the manual layer to your Lambda function: `arn:aws:lambda:<region>:115813213817:layer:dash0-extension-manual:<version>`.
-2. Configure your OTLP trace exporter to send to `http://127.0.0.1:9009/v1/traces`.
-3. If exporting metrics, configure your OTLP metric exporter to send to `http://127.0.0.1:9009/v1/metrics`.
-4. Make sure to flush all telemetry before the Lambda invocation completes (e.g., in a response hook or before returning the response).
+2. Set `AWS_LAMBDA_EXEC_WRAPPER=/opt/wrapper`. This is **required** (see [Required](#required)): it routes the Lambda Runtime API through the extension so it can correlate spans to the current invocation. Without it the extension cannot determine the invocation ID and traces are dropped with `[DASH0] /v1/traces has no invocation IDs, discarding trace`.
+3. Configure your OTLP trace exporter to send to `http://127.0.0.1:9009/v1/traces`.
+4. If exporting metrics, configure your OTLP metric exporter to send to `http://127.0.0.1:9009/v1/metrics`.
+5. Make sure to flush all telemetry before the Lambda invocation completes (e.g., in a response hook or before returning the response).
 
 
 ## Enrichment Attributes