Implement the ExecutedActionMetadata (timestamps)

Context

It would be useful to be able to keep track of execution timestamps. The REAPI defines ExecutedActionMetadata (see here) containing timestamps for various events:

• queued (server side).
• worker_start and worker_completed (server side).
• input_fetch_start and input_fetch_completed (bot side).
• execution_start and execution_completed (bot side).
• output_upload_start and output_upload_completed (bot side).

Task Description

• Implement server-side timestamps.
• Implement bot-side timestamps:
  • Implement timestamps for the dummy bot.
  • Implement timestamps for the host-tools bot.
  • Implement timestamps for the buildbox bot.
• Have bgd operation commands report metrics when they are available.

Acceptance Criteria

• bgd operation wait returns time metrics after operation execution completed.