Add nodejs tutorial to Observability docs

Adds an tutorial for instrumenting a NodeJS project
with Observability traces.

Changelog: added

doc/tutorials/observability/observability_nodejs_tutorial.md

+---
+stage: Monitor
+group: Observability
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://handbook.gitlab.com/handbook/product/ux/technical-writing/#assignments
+---
+
+# Tutorial: Use GitLab Observability with a NodeJS application
+
+DETAILS:
+**Tier:** Ultimate
+**Offering:** GitLab.com
+**Status:** Beta
+
+> - Observability tracing [introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/124966) in GitLab 16.2 [with a flag](../../administration/feature_flags.md) named `observability_tracing`. Disabled by default.
+
+FLAG:
+The availability of this feature is controlled by a feature flag.
+For more information, see the history.
+This feature is available for testing, but not ready for production use.
+
+In this tutorial, you'll learn how to configure, instrument, and monitor a NodeJS application using GitLab Observability features.
+
+## Before you begin
+
+Take a moment and make sure you have the following:
+
+- A GitLab Ultimate subscription for GitLab.com
+- A local installation of NodeJS
+- Basic knowledge of Git, NodeJS, JavaScript, and the core concepts of [OpenTelemetry](https://opentelemetry.io/)
+
+## Create a new GitLab project
+
+First, create a new GitLab project and a corresponding access token.
+This tutorial uses the project name `nodejs-O11y-tutorial`.
+
+1. On the left sidebar, at the top, select **Create new** (**{plus}**) and **New project/repository**.
+1. Select **Create from template**.
+1. Select **Use template** for NodeJS Express.
+1. Enter the project details.
+   - In the **Project name** field, enter `nodejs-O11y-tutorial`.
+1. Select **Create project**.
+1. In the `nodejs-O11y-tutorial` project, on the left sidebar, select **Settings > Access tokens**.
+1. Create a new access token with the Owner role and the `read_api` and `write_observability` scopes. Store the token value somewhere safe—you'll need it later.
+
+## Instrument your NodeJS application
+
+Next, we need to instrument the NodeJS application.
+
+1. Ensure you have [NodeJS](https://nodejs.org/en) installed by running the following:
+
+   ```shell
+   node -v
+   ```
+
+1. Clone the `nodejs-O11y-tutorial` project and `cd` to the `nodejs-O11y-tutorial` directory.
+1. Install the dependencies by running:
+
+   ```shell
+   npm install
+   ```
+
+1. Run the application:
+
+   ```shell
+   PORT=8080 node server.js
+   ```
+
+1. In a web browser, visit `http://localhost:8080` and make sure the application is running correctly.
+1. Add the OpenTelemetry packages:
+
+   ```shell
+   npm install --save @opentelemetry/api \
+     @opentelemetry/auto-instrumentations-node
+   ```
+
+1. Find your group ID:
+   1. On the left sidebar, select **Search or go to** and find the top-level group with the `nodejs-O11y-tutorial` project. For example, if your project URL is `https://gitlab.com/tankui/observability/nodejs-O11y-tutorial`, the top-level group is `tanuki`.
+   1. On the group overview page, in the upper-right corner, select **Actions** (**{ellipsis_v}**).
+   1. Select **Copy group ID**. Save the copied ID for later.
+1. Find your project ID:
+   1. On the `nodejs-O11y-tutorial` project overview page, in the upper-right corner, select **Actions** (**{ellipsis_v}**).
+   1. Select **Copy project ID**. Save the copied ID for later.
+
+1. Configure and run your project with instrumentation:
+
+   ```shell
+   env OTEL_TRACES_EXPORTER="otlp" \
+   OTEL_EXPORTER_OTLP_ENDPOINT="https://observe.gitlab.com/v3/{{GROUP_ID}}/{{PROJECT_ID}}/ingest" \
+   OTEL_EXPORTER_OTLP_HEADERS="PRIVATE-TOKEN={{ACCESS_TOKEN}}" \
+   OTEL_SERVICE_NAME="nodejs-O11y-tutorial" \
+   OTEL_LOG_LEVEL="debug" \
+   NODE_OPTIONS="--require @opentelemetry/auto-instrumentations-node/register" \
+   PORT=8080 node server.js
+   ```
+
+   Be sure to replace the `GROUP_ID`, `PROJECT_ID`, and `ACCESS_TOKEN` with the values you obtained earlier.
+
+## View traces
+
+Now that you have an application configured to use Observability tracing,
+you can view exported traces on GitLab.com.
+
+To view exported traces:
+
+1. Start the `nodejs-O11y-tutorial` application with instrumentation again.
+1. Visit `http://localhost:8080/` and perform some actions in the application.
+1. In the `nodejs-O11y-tutorial` project, on the left sidebar, select **Monitor > Traces**.
+   If everything is working correctly, you should see a trace for each request.
+
+   ![Metrics UI](img/nodejs_metrics_ui.png)
+
+1. Optional. Select a trace to view its span.
+
+   ![Traces UI](img/nodejs_single_trace.png)
+
+Congratulations! You successfully created an application, configured it to use GitLab Observability features, and examined the traces the application created. You can continue to experiment with this application, or try configuring a more complex application to export traces.
+
+Remember that Observability Tracing is not yet ready for production use. There is no official support for logs or metrics using the OpenTelemetry collector with a NodeJS application.

doc/tutorials/observability/observability_rails_tutorial.md

@@ -126,11 +126,11 @@ To view exported traces:
 1. In the `animals` project, on the left sidebar, select **Monitor > Traces**.
    If everything is working correctly, you should see a trace for each controller action.
 
-   ![Metrics UI](img/metrics_ui.png)
+   ![Metrics UI](img/rails_metrics_ui.png)
 
 1. Optional. Select a trace to view its span.
 
-   ![Traces UI](img/single_trace.png)
+   ![Traces UI](img/rails_single_trace.png)
 
 Congratulations! You successfully created an application, configured it to use GitLab Observability features, and examined the traces the application created. You can continue to experiment with this toy application, or try configuring a more complex application to export traces.