Document Apollo client link

packages/react-native-apollo-client/README.md

@@ -1 +1,95 @@
 # React-Native Monitoring for @apollo/client
+
+## Overview
+
+Real User Monitoring (RUM) allows you to monitor your GraphQL resources.
+
+You can perform the following:
+
+-   Identify GraphQL queries and mutations
+-   Identify GraphQL variables used in queries and mutations
+
+RUM supports GraphQL requests created using [@apollo/client][2].
+
+## Setup
+
+### Prerequisites
+
+Set up the RUM Browser SDK on the web page you want rendered on your mobile React Native application. For more information, see [RUM Browser Monitoring][1].
+
+Add `@apollo/client` to your application following the [official installation documentation][3].
+
+### Instrument your ApolloClient
+
+#### Migrate to HttpLink
+
+If you initialize your ApolloClient with the `uri` parameter, initialize it with a `HttpLink`:
+
+```javascript
+import { ApolloClient, HttpLink } from '@apollo/client';
+
+// before
+const apolloClient = new ApolloClient({
+    uri: 'https://my.api.com/graphql'
+});
+
+// after
+const apolloClient = new ApolloClient({
+    link: new HttpLink({ uri: 'https://my.api.com/graphql' })
+});
+```
+
+#### Use the Datadog Apollo Client Link to collect information
+
+Import `DatadogLink` from `@datadog/mobile-react-native-apollo-client` and use it in your ApolloClient initialization:
+
+```javascript
+import { ApolloClient, from, HttpLink } from '@apollo/client';
+import { DatadogLink } from '@datadog/mobile-react-native-apollo-client';
+
+const apolloClient = new ApolloClient({
+    link: from([
+        new DatadogLink(),
+        new HttpLink({ uri: 'https://my.api.com/graphql' }) // always in last position
+    ])
+});
+```
+
+For more information on Apollo Client Links, refer to the [official documentation][4].
+
+### Removing GraphQL information
+
+Use a `resourceEventMapper` in your Datadog configuration to remove sensitive data from GraphQL variables:
+
+```javascript
+const datadogConfiguration = new DatadogProviderConfiguration(
+    '<CLIENT_TOKEN>',
+    '<ENVIRONMENT_NAME>',
+    '<RUM_APPLICATION_ID>',
+    true,
+    true,
+    true
+);
+
+datadogConfiguration.resourceEventMapper = event => {
+    // Variables are stored in event.context['_dd.graphql.variables'] as a JSON string when present
+    if (event.context['_dd.graphql.variables']) {
+        const variables = JSON.parse(event.context['_dd.graphql.variables']);
+        if (variables.password) {
+            variables.password = '***';
+        }
+        event.context['_dd.graphql.variables'] = JSON.stringify(variables);
+    }
+
+    return event;
+};
+```
+
+## Access your GraphQL resources information
+
+The information is not yet surfaced on the Datadog platform.
+
+[1]: https://docs.datadoghq.com/real_user_monitoring/browser/#npm
+[2]: https://www.apollographql.com/docs/react/
+[3]: https://www.apollographql.com/docs/react/get-started
+[4]: https://www.apollographql.com/docs/react/api/link/introduction/