Add small doc about running a standalone agent in otel mode

docs/en/ingest-management/elastic-agent/otel-agent-transform.asciidoc

@@ -3,18 +3,78 @@
 
 preview::[]
 
-If you have a currently installed {agent}, you change it to run as an <<otel-agent,OTel Collector>>.
+If you have a currently installed standalone {agent}, it can be configured to run as an <<otel-agent,OTel Collector>>.
 
-To transform {agent} to an OTel Collector:
+This will allow to run Elastic Agent as a service running Otel Collector with a given configuration.
 
-. Some instruction.
+In order to configure an installed standalone {agent} to run as an OTel Collector it's enough to include a valid <<otel-agent,OTel Collector>> configuration in `elastic-agent.yml`.
 
-. Some other instruction. Run the `elastic-agent -transform-otel` command:
+
+=== Example: configure {agent} to ingest host logs and metrics into Elasticsearch using Otel Collector
+
+Prerequisites:
+
+. A suitable API Key is available for authenticating on Elasticsearch
+. An installed standalone {agent}
+. A valid OTel Collector configuration, in this example we are going to use an included OTel sample configuration in `otel_samples/platformlogs_hostmetrics.yml`
+link:https://github.com/elastic/elastic-agent/blob/main/internal/pkg/otel/samples/linux/platformlogs_hostmetrics.yml[Linux example]
+link:https://github.com/elastic/elastic-agent/blob/main/internal/pkg/otel/samples/darwin/platformlogs_hostmetrics.yml[MacOS example]
+
+Steps:
+
+. Create a directory where Otel Collector can save its state (in this example we use `<{agent} install directory>/data/otelcol`)
+. Open `<{agent} install directory>/otel_samples/platformlogs_hostmetrics.yml` with your favourite editor
+. Set environment details to be used by OTel Collector
+* [Option 1] Define environment variables for the {agent} service:
+** `ELASTIC_ENDPOINT`: URL of Elasticsearch instance where data is going to be ingested
+** `ELASTIC_API_KEY`: API Key for Elasticsearch authentication
+** `STORAGE_DIR`: directory where Otel Collector can persist its state
+* [Option 2] Replace the environment variable references in the sample configuration with the corresponding values
+** `${env:ELASTIC_ENDPOINT}`: URL of Elasticsearch instance where data is going to be ingested
+** `${env:ELASTIC_API_KEY}`: API Key for Elasticsearch authentication
+** `${env:STORAGE_DIR}`: directory where Otel Collector can persist its state
+. Save the opened OTel configuration as `elastic-agent.yml`, overwriting the default configuration of the installed agent
+. Verify that the new configuration is correctly applied by agent running a `status` command:
 +
 [source,shell]
 ----
-elastic-agent -switch-me-to-otel-please
+elastic-agent status
 ----
-
-
-
+Otel Collector running configuration should appear under `elastic-agent` key (note the `extensions` and `pipeline` keys)
++
+[source,shell]
+----
+┌─ fleet
+│  └─ status: (STOPPED) Not enrolled into Fleet
+└─ elastic-agent
+   ├─ status: (HEALTHY) Running
+   ├─ extensions
+   │  ├─ status: StatusOK
+   │  └─ extension:file_storage
+   │     └─ status: StatusOK
+   ├─ pipeline:logs/platformlogs
+   │  ├─ status: StatusOK
+   │  ├─ exporter:elasticsearch/otel
+   │  │  └─ status: StatusOK
+   │  ├─ processor:resourcedetection
+   │  │  └─ status: StatusOK
+   │  └─ receiver:filelog/platformlogs
+   │     └─ status: StatusOK
+   └─ pipeline:metrics/hostmetrics
+      ├─ status: StatusOK
+      ├─ exporter:elasticsearch/ecs
+      │  └─ status: StatusOK
+      ├─ processor:attributes/dataset
+      │  └─ status: StatusOK
+      ├─ processor:elasticinframetrics
+      │  └─ status: StatusOK
+      ├─ processor:resource/process
+      │  └─ status: StatusOK
+      ├─ processor:resourcedetection
+      │  └─ status: StatusOK
+      └─ receiver:hostmetrics/system
+         └─ status: StatusOK
+----
++
+. Congratulations! Host logs and metrics are now being collected and ingested by the {agent} service running an OTel Collector instance.
+For further details about OpenTelemetry collector components supported by Elastic Agent, refer to link:https://github.com/elastic/elastic-agent/tree/main/internal/pkg/otel#components[Elastic Distribution for OpenTelemetry Collector README]