docs: explain admin interface and stats

docs/root/development/features/admin_interface.rst

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+.. _admin_interface:
+
+Admin Interface
+====================
+
+You can read more about this Envoy's feature and how to use it [here|https://www.envoyproxy.io/docs/envoy/latest/operations/admin].
+By default, Envoy Mobiles does not come with an admin endpoint enabled. You can enable it in the following way.
+
+**Kotlin**::
+
+  builder.enableAdminInterface()
+
+**Swift**::
+
+  builder.enableAdminInterface()
+
+Envoy Mobile runs its administration interface on port 9901.
+
+One of the most common use cases for the interface is to check the state of all of the Envoy stats at a given moment. You can do this by running the following command:
+
+```
+> curl localhost:9901/stats
+```
+
+Please note that to make this command work when you are running an example app using Android emulator you need to set up port forwarding first:
+```
+> adb forward tcp:9901 tcp:9901
+```

docs/root/development/features/stats.rst

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+.. _stats:
+
+Stats
+============
+
+Envoy and by extension Envoy Mobile implements a comprehensive set of stats
+that may be used to monitor the health of the library and for debugging purposes.
+
+Envoy Mobile does not enable all of the Envoy stats by default. Instead, it
+implements an allow list of stats that are allowed to be created on the client-side
+and ingested to a remote service. The allow list can be found under `stats_config`
+in the `library/common/config.cc` file.
+
+The original idea behind not enabling all Envoy stats by default was to limit
+the usage of resources on the client:
+
+1. Lower the usage of memory and CPU resources by creating and maintaining the lower number of instances of stats
+2. Reduce the cell data plan usage by ingesting less data to a remote service