docs: add how-to guide on integrating third-party services

docs/how_tos/integrating-external-services.rst

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+#####################################################################
+How to: Integrating with third-party services for logging & analytics
+#####################################################################
+
+Context
+***********************
+
+Service classes
+==========
+
+The ``frontend-platform`` library consists of several modules (e.g., logging, analytics, authentication, etc.) consumed by micro-frontends (MFEs) throughout the Open edX platform. The library has been architected in such a way that enables consumers to override default, base behavior of modules by overriding the default implementations of some module interfaces.
+
+While modules (e.g., ``logging`` and ``analytics``) define base service classes, the ``initialize`` function exposes a mechanism for consumers to override the base service classes during MFE initialization, as needed to extend the library behavior or integrate with other external services.::
+
+  // override default services via ``initialize`` in MFE entrypoints
+  initialize({
+    loggingService = CustomLoggingService,
+    analyticsService = CustomAnalyticsService,
+    // ...
+  });
+
+  // override default services via private ``env.config``
+  const config = {
+    loggingService: CustomLoggingService,
+    analyticsService: CustomAnalyticsService,
+  };
+  export default config;
+
+Each module with a service class exposes an interface and subsequently implements a base service class for the defined interface. For example, ``src/logging/interface.js`` defines the following interface:
+
+- ``logInfo``
+- ``logError``
+- ``setCustomAttribute``
+
+Regarding analytics, ``src/analytics/interface.js`` defines the service as having the following functions:
+
+- ``sendTrackingLogEvent``
+- ``identifyAuthenticatedUser``
+- ``identifyAnonymousUser``
+- ``sendTrackEvent``
+- ``sendPageEvent``
+
+When overriding the base service classes (e.g., a custom ``LoggingService`` class), each of the required, defined interface functions must be implemented. The implementation of the interface is currently verified via ``PropTypes.checkPropTypes`` during the configuration/instantiation of the service class.
+
+Other mechanisms to integrate third-party services
+==========
+
+
+``loadExternalScripts``
+------------
+
+During MFE initialization, the ``initialize`` function accepts an optional ``externalScripts`` argument, representing a list of external "loader" classes instantiated via ``loadExternalScripts``::
+
+  initialize({
+    externalScripts: [GoogleAnalyticsLoader],
+    // ...
+  })
+
+By default, ``externalScripts`` contains a ``GoogleAnalyticsLoader``, but may be extended to include other loader classes, too.
+
+Open questions:
+
+#. Why isn't ``GoogleAnalyticsLoader`` treated as an overridden ``analyticsService``?
+#. Under what cirumstances *should* ``externalScripts`` be used versus overriding/extending the default ``loggingService`` vs. ``analyticsService``?
+#. Is it possible to use ``externalScripts`` without modifying ``openedx`` MFE code (e.g., does it require a fork?)
+#. Any scripts implemented via ``externalScripts`` will not be called via the service functions implemented throughout MFEs. For instance, what if an Open edX instance wants Google Analytics events for the instrumented ``sendTrackEvent`` usages throughout the platform?
+
+(Proposed) ``useComponentPropOverrides`` & ``withComponentPropOverrides``
+------------
+
+Neither of the above approaches to integrate third-party, external services into Open edX MFEs support passing vendor-specific HTML attributes or class names to specific rendered components. For example, to suppress/unsuppress PII from session replays in tools like Datadog/Hotjar, it necessitates passing a custom prop to the component that may not be hardcoded into ``openedx`` repositories (e.g., ``data-dd-privacy`` for Datadog).
+
+A `proposed solution <https://github.com/openedx/frontend-platform/pull/723>`_ to this challenge is the addition of supporting the ability to override props for specific components via private configuration through the use of ``useComponentPropOverrides`` and/or ``withComponentPropOverrides``::
+
+  // example env.config
+  const config = {
+    componentPropOverrides: {
+    targets: {
+      example: {
+        'data-dd-privacy': 'mask', // Custom `data-*` attribute (e.g., Datadog)
+        onClick: (e) => console.log('custom event handler'),
+      },
+    },
+  };
+  export default config;
+
+By default, ``useComponentPropOverrides`` and ``withComponentPropOverrides`` supports any custom `className` or `data-*` props. However, specific components may allow other prop types to be overridden applied by supplying the ``allowedPropNames`` and/or ``allowsDataAttributes`` options::
+
+  const ExampleComponentWithDefaultPropOverrides = ({ children, ...rest }) => {
+    const propOverrides = useComponentPropOverrides('example', rest);
+    return <span {...propOverrides}>{children}</span>;
+  });
+  
+  const ExampleComponentWithAllowedPropOverrides = ({ children, ...rest }) => {
+    const propOverrides = useComponentPropOverrides('example', rest, {
+      allowedPropNames: ['className', 'style', 'onClick'],
+      allowsDataAttributes: true,
+    });
+    return <span {...propOverrides}>{children}</span>;
+  });
+
+Performance considerations
+***********************
+
+Performance implications should be considered while integrating with other services.Any third-party, external vendor scripts (e.g., Segment, Google Analytics, Datadog, etc.) loaded via a service class (e.g., ``SegmentAnalyticsService``, ``externalScripts``) are injecting ``<script>`` tags into the HTML document *after* much of the MFE's application code has been downloaded/parsed. That is, all JS needed for frontend-platform's MFE initialization must be downloaded/evaluated first *before* downloading any external scripts.
+
+The default/base ``NewRelicLoggingService`` is currently an exception in that it injects the New Relic JS `<script>` tag during the Webpack build process via ``HtmlWebpackNewRelicPlugin``, whereby the New Relic `<script>` may be downloaded earlier in the initial page load, without being blocked on parsing frontend-platform's JS. In a similar vein, many MFEs integrate with Optimizely by hardcoding its associated ``<script>`` element in the ``public/index.html`` file for the MFE (not open-source friendly).