Update telemetry documentation for occurrence metadata

Reflect that :new and :unresolved error events now include the
occurrence in their metadata, and document the nullable semantics
for manual unresolves.

Author: gaggle

lib/error_tracker/telemetry.ex

@@ -31,17 +31,20 @@ defmodule ErrorTracker.Telemetry do
   Each event is emitted with some measures and metadata, which can be used to
   receive information without having to query the database again:
 
-  | event                                   | measures       | metadata                          |
-  | --------------------------------------- | -------------- | ----------------------------------|
-  | `[:error_tracker, :error, :new]`        | `:system_time` | `:error`                          |
-  | `[:error_tracker, :error, :unresolved]` | `:system_time` | `:error`                          |
-  | `[:error_tracker, :error, :resolved]`   | `:system_time` | `:error`                          |
-  | `[:error_tracker, :occurrence, :new]`   | `:system_time` | `:occurrence`, `:error`, `:muted` |
+  | event                                   | measures       | metadata                                        |
+  | --------------------------------------- | -------------- | ----------------------------------------------- |
+  | `[:error_tracker, :error, :new]`        | `:system_time` | `:error`, `:occurrence`                         |
+  | `[:error_tracker, :error, :unresolved]` | `:system_time` | `:error`, `:occurrence` (nullable)              |
+  | `[:error_tracker, :error, :resolved]`   | `:system_time` | `:error`                                        |
+  | `[:error_tracker, :occurrence, :new]`   | `:system_time` | `:occurrence`, `:error`, `:muted`               |
 
   The metadata keys contain the following data:
 
   * `:error` - An `%ErrorTracker.Error{}` struct representing the error.
   * `:occurrence` - An `%ErrorTracker.Occurrence{}` struct representing the occurrence.
+    For `:new` error events this is the first occurrence. For `:unresolved` events this
+    is the occurrence that triggered the state change, or `nil` when the error was manually
+    unresolved from the UI.
   * `:muted` - A boolean indicating whether the error is muted or not.
   """