Expand OpenTelemetry documentation for Lambdas and ADOT

Added information about OpenTelemetry for Lambdas and ADOT.

Author: monodot

_articles/opentelemetry.md

@@ -18,3 +18,21 @@ lede: Vendor-neutral telemetry for infra and applications.
 - **Head-based sampling** is _"where the sampling decision is made at the **beginning** of a request when the root span begins processing"_
 - **Tail sampling** is _"where the sampling decision is made **after** all the spans in a request have been completed"_
 
+## Lambdas/serverless
+
+opentelemetry-lambda:
+
+- See https://github.com/open-telemetry/opentelemetry-lambda/releases
+- ARNs like:
+  - `arn:aws:lambda:<region>:184161586896:layer:opentelemetry-javaagent-0_17_0:1`
+  - `arn:aws:lambda:<region>:184161586896:layer:opentelemetry-nodejs-0_19_0:1`
+  - `arn:aws:lambda:<region>:184161586896:layer:opentelemetry-collector-<amd64|arm64>-0_12_0:1` (a stripped-down version of OTel Collector inside an AWS Extension Layer)
+
+ADOT (AWS Distribution of OpenTelemetry):
+
+- ADOT is a **downstream repo of opentelemetry-lambda**
+- AWS-managed OpenTelemetry Lambda layers that are preconfigured for use with AWS services and bundle the reduced ADOT Collector
+- ARNs like:
+  - `arn:aws:lambda:ca-central-1:901920570463:layer:aws-otel-nodejs-amd64-ver-1-18-0:1` - "legacy" layer which includes an embedded collector.
+  - `arn:aws:lambda:eu-central-1:615299751070:layer:AWSOpenTelemetryDistroPython:21` - new-style layer which works with CloudWatch **only**. 
+