Update docs for encryption support

docs/SUMMARY.md

@@ -25,4 +25,5 @@
 
 - [Provisioning](./advanced/provisioning.md)
 - [Workers](./advanced/workers.md)
+- [🔒 Encryption](./advanced/encryption.md)
 - [🪀 Widgets](./advanced/widgets.md)

docs/advanced/encryption.md

@@ -0,0 +1,23 @@
+Encryption
+=======
+
+Hookshot supports end-to-bridge encryption via [MSC3202](https://github.com/matrix-org/matrix-spec-proposals/pull/3202). As such, encryption requires hookshot to be connected to a homeserver that supports that MSC, such as [Synapse](#running-with-synapse).
+
+## Enabling encryption in Hookshot
+
+In order for hookshot to use encryption, it must be configured as follows:
+- The `encryption.storagePath` setting must point to a directory that hookshot has permissions to write files into.
+- [Workers](./workers.md) must be enabled.
+
+Also ensure that hookshot's appservice registration file contains every line from `registration.sample.yml` that appears after the `If enabling encryption` comment. Note that changing the registration file may require restarting the homeserver that hookshot is connected to.
+
+## Running with Synapse
+
+[Synapse](https://github.com/matrix-org/synapse/) has functional support for MSC3202 as of [v1.63.0](https://github.com/matrix-org/synapse/releases/tag/v1.63.0). To enable it, add the following section to Synapse's configuration file (typically named `homeserver.yaml`):
+
+```yaml
+experimental_features:
+  msc3202_device_masquerading: true
+  msc3202_transaction_extensions: true
+  msc2409_to_device_messages_enabled: true
+```

docs/metrics.md

@@ -37,6 +37,7 @@ Below is the generated list of Prometheus metrics for Hookshot.
 | matrix_api_calls | The number of Matrix client API calls made | method |
 | matrix_api_calls_failed | The number of Matrix client API calls which failed | method |
 | matrix_appservice_events | The number of events sent over the AS API |  |
+| matrix_appservice_decryption_failed | The number of events sent over the AS API that failed to decrypt |  |
 ## feed
 | Metric | Help | Labels |
 |--------|------|--------|