yabeda-hotcell counts a call interrupted by an exception as code="ok" #93

When an exception escapes a call, such as the application's own request timeout, yabeda-hotcell counts the call as code="ok" and records a perform of 0 seconds. The perform.hot_cell event then carries exception and no code, and record_perform falls back to "ok" whenever code is missing.

HotCell::LogSubscriber already handles this case: it writes the exception's class and no code.

To reproduce in an application with yabeda-hotcell 0.6.0 installed:

ActiveSupport::Notifications.instrument("perform.hot_cell", cell: "active_storage", operation: "health.echo") do
  raise Timeout::Error
end
# Yabeda.hotcell.requests: {cell: "active_storage", operation: "health.echo", code: "ok", cause: ""} => 1

Before 0.6.0, these calls carried no cell or operation and landed in a series of their own. Since #78 added both, they inflate ok for the real cell and operation.

Proposal: count these calls under a code of their own, such as exception, and skip the perform observation, because the cell measured nothing.

I found this while upgrading HEY (basecamp/haystack#8917). HEY's own Yabeda module, which that PR deletes, had the same bug.