fix(google-cloud): format_affected_user/1 with non-binary values (#161)

### 🐞 Problem  
The `google_cloud` formatter crashes when it receives a crash log where
the `user context` contains values that are not binaries (e.g.,
integers). This happens because the formatter uses the `<>` operator for
string concatenation, which raises an error when used with non-binary
types.

**Truncated example of a crashed log:**

```bash
2025-01-01T01:01:01.000000+00:00 error: FORMATTER CRASH: {string,<<"** (Postgrex.Error) ERROR 428...
```

### ✅ Solution  
This PR replaces the use of the `<>` operator with `Enum.join/2` when
formatting key-value pairs. This approach safely converts all values to
strings before concatenation, preventing crashes when values are not
binaries.

Author: raulpe7eira

lib/logger_json/formatters/google_cloud.ex

@@ -329,10 +329,10 @@ defmodule LoggerJSON.Formatters.GoogleCloud do
     end
   end
 
-  defp format_affected_user(%{user_id: user_id}), do: "user:" <> user_id
-  defp format_affected_user(%{identity_id: identity_id}), do: "identity:" <> identity_id
-  defp format_affected_user(%{actor_id: actor_id}), do: "actor:" <> actor_id
-  defp format_affected_user(%{account_id: account_id}), do: "account:" <> account_id
+  defp format_affected_user(%{user_id: user_id}), do: "user:#{user_id}"
+  defp format_affected_user(%{identity_id: identity_id}), do: "identity:#{identity_id}"
+  defp format_affected_user(%{actor_id: actor_id}), do: "actor:#{actor_id}"
+  defp format_affected_user(%{account_id: account_id}), do: "account:#{account_id}"
   defp format_affected_user(_meta), do: nil
 
   defp format_stacktrace(stacktrace) do

test/logger_json/formatters/google_cloud_test.exs

@@ -338,7 +338,7 @@ defmodule LoggerJSON.Formatters.GoogleCloudTest do
              """
   end
 
-  test "logs exception user context" do
+  test "logs exception user context with binary values" do
     Logger.metadata(crash_reason: {{:EXIT, self()}, :foo})
 
     # The keys are applied in the order of their precedence
@@ -362,6 +362,30 @@ defmodule LoggerJSON.Formatters.GoogleCloudTest do
     end)
   end
 
+  test "logs exception user context with non-binary values" do
+    Logger.metadata(crash_reason: {{:EXIT, self()}, :foo})
+
+    # The keys are applied in the order of their precedence
+    [:user_id, :identity_id, :actor_id, :account_id]
+    |> Enum.reverse()
+    |> Enum.reduce([], fn key, metadata ->
+      metadata = Keyword.put(metadata, key, 123)
+      Logger.metadata(metadata)
+
+      log_entry =
+        capture_log(fn ->
+          Logger.debug("Hello")
+        end)
+        |> decode_or_print_error()
+
+      [entity, _id] = key |> Atom.to_string() |> String.split("_")
+
+      assert log_entry["context"]["user"] == "#{entity}:123"
+
+      metadata
+    end)
+  end
+
   test "logs http context" do
     conn =
       Plug.Test.conn("GET", "/", "")