[Backport 7.x] Added new observability events (#1370)

Co-authored-by: Tomas Della Vedova <delvedor@users.noreply.github.com>

docs/observability.asciidoc

@@ -49,6 +49,15 @@ client.on('response', (err, result) => {
 The client emits the following events:
 [cols=2*]
 |===
+|`serialization`
+a|Emitted before starting serialization and compression. If you want to measure this phase duration, you should measure the time elapsed between this event and `request`.
+[source,js]
+----
+client.on('serialization', (err, result) => {
+  console.log(err, result)
+})
+----
+
 |`request`
 a|Emitted before sending the actual request to {es} _(emitted multiple times in case of retries)_.
 [source,js]
@@ -58,6 +67,15 @@ client.on('request', (err, result) => {
 })
 ----
 
+|`deserialization`
+a|Emitted before starting deserialization and decompression. If you want to measure this phase duration, you should measure the time elapsed between this event and `response`. _(This event might not be emitted in certain situations)_.
+[source,js]
+----
+client.on('deserialization', (err, result) => {
+  console.log(err, result)
+})
+----
+
 |`response`
 a|Emitted once {es} response has been received and parsed.
 [source,js]
@@ -87,7 +105,7 @@ client.on('resurrect', (err, result) => {
 
 |===
 
-The values of `result` in `request`, `response` and `sniff` will be:
+The values of `result` in `serialization`, `request`, `deserialization`, `response` and `sniff` will be:
 
 [source,ts]
 ----
@@ -127,6 +145,29 @@ request: {
 };
 ----
 
+[discrete]
+==== Events order
+
+The event order is described in the following graph, in some edge cases, the order is not guaranteed.
+You can find in https://github.com/elastic/elasticsearch-js/blob/master/test/acceptance/events-order.test.js[`test/acceptance/events-order.test.js`] how the order changes based on the situation.
+
+[source]
+----
+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
+----
+
 
 [discrete]
 === Correlation id