Add README for LangGraph sample

Author: aabmass

samples/langgraph-sql-agent/README.md

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+# OpenTelemetry LangGraph instrumentation example
+
+<!-- TODO: link to devsite doc once it is published -->
+
+This sample is a LangGraph agent instrumented with OpenTelemetry to send traces and logs
+including GenAI prompts and responses to Google Cloud Observability.
+
+The Agent is a SQL expert that has full access to an ephemeral SQLite database. The database is
+initially empty. It is built with the the LangGraph prebuilt [ReAct
+Agent](https://langchain-ai.github.io/langgraph/agents/agents/#basic-configuration#code) and the
+[SQLDatabaseToolkit](https://python.langchain.com/docs/integrations/tools/sql_database/).
+
+## Permissions
+
+This sample writes to Cloud Logging, Cloud Monitoring, and Cloud Trace. Grant yourself the
+following roles to run the example:
+- `roles/logging.logWriter` – see https://cloud.google.com/logging/docs/access-control#permissions_and_roles
+- `roles/monitoring.metricWriter` – see https://cloud.google.com/monitoring/access-control#predefined_roles
+- `roles/cloudtrace.agent` – see https://cloud.google.com/trace/docs/iam#trace-roles
+
+## Running the example
+
+```sh
+git clone https://github.com/GoogleCloudPlatform/opentelemetry-operations-python.git
+cd opentelemetry-operations-python/samples/langgraph-sql-agent
+
+# Capture GenAI prompts and responses
+export OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT=true
+# Capture application logs automatically
+export OTEL_PYTHON_LOGGING_AUTO_INSTRUMENTATION_ENABLED=true
+uv run main.py
+```
+
+## Viewing the results
+
+Similarly, to view the generated traces in the GCP console, use the Trace Explorer.