Added new observability events

[delvedor]

This pr adds two new events, serialization and deserialization.
The event order is described in the following graph, in some edge cases, the order is not guaranteed.
You can find in test/acceptance/events-order.test.js how the order changes based on the situation.

serialization
  │
  │ (serialization and compression happens between those two events)
  │
  └─▶ request
        │
        │ (actual time spent over the wire)
        │
        └─▶ deserialization
              │
              │ (deserialization and decompression happens between those two events)
              │
              └─▶ response

cc @trentm

[EamonnTP]

@elasticmachine, run elasticsearch-ci/docs