Log every HotCell call to the Rails log by default

[flavorjones]

Motivation

The client publishes the perform.hot_cell Active Support notification on every call, success or failure (client.rb). Its payload carries the cell, the operation, the failure code and cause, the tool's stderr, the byte counts, and the time the cell measured (publish).

Nothing in the gem turns that event into a log line. An application that wanted one line per call had to write its own subscriber, and each application that adopted HotCell wrote the same one.

Details

• HotCell::LogSubscriber is an ActiveSupport::LogSubscriber. It writes one info line per call:

  HotCell (41.2ms) {"cell":"images","operation":"active_storage.transformers.image.vips","code":"ok","perform_ms":38,"duration_ms":41.2,"bytes_in":20480,"bytes_out":8192}
  

  A failed call adds cause and stderr when it has them. A call that an exception interrupts, such as the application's own request timeout, logs the exception's class in place of the code. stderr is text a tool wrote while reading an untrusted file. It goes only inside the JSON, which is ASCII-only, so neither a newline nor a Unicode line separator in it can forge a log line.

• The railtie attaches the subscriber at boot, so a Rails application logs to Rails.logger with no setup. An application turns it off with HotCell::LogSubscriber.detach_from :hot_cell. Without Rails, an application requires hot_cell/log_subscriber, calls attach_to :hot_cell, and sets ActiveSupport::LogSubscriber.logger.

• An application that already logs this event should delete its own subscriber, or it will log each call twice. The CHANGELOG says so under "Upgrading".

Fixes #10

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