Update events

[delvedor]

This pr adds the prepare-request event, which comes very handy if someone needs to track when a request has started (the request event is emitted after the serialization and building the headers).

prepare-request emits an object with the following keys:

• meta: all the metadata about the request
• params: the parameters the user has passed to the API
• options: the custom options for the request

In addition, if the serialization fails, the request event will be emitted with an error.

The event flow now is:

• prepare-request
• request
• sniff (not guaranteed)
• resurrect (not guaranteed)
• response

Related: #834 elastic/apm-agent-nodejs#968

[watson]

LGTM

[stale]

We understand that this might be important for you, but this issue has been automatically marked as stale because it has not had recent activity either from our end or yours.
It will be closed if no further activity occurs, please write a comment if you would like to keep this going.

Note: in the past months we have built a new client, that has just landed in master. If you want to open an issue or a pr for the legacy client, you should do that in https://github.com/elastic/elasticsearch-js-legacy

[stale]

We understand that this might be important for you, but this issue has been automatically marked as stale because it has not had recent activity either from our end or yours.
It will be closed if no further activity occurs, please write a comment if you would like to keep this going.

Note: in the past months we have built a new client, that has just landed in master. If you want to open an issue or a pr for the legacy client, you should do that in https://github.com/elastic/elasticsearch-js-legacy