X-Opaque-Id support (#997)

* Added X-Opaque-Id support

* Updated type definitions

* Updated test

* Updated docs

docs/observability.asciidoc

@@ -248,3 +248,46 @@ child.search({
   if (err) console.log(err)
 })
 ----
+
+=== X-Opaque-Id support
+To improve the overall observability, the client offers an easy way to configure the `X-Opaque-Id` header. If you set the `X-Opaque-Id` in a specific request, this will allow you to discover this identifier in the https://www.elastic.co/guide/en/elasticsearch/reference/master/logging.html#deprecation-logging[deprecation logs], help you with https://www.elastic.co/guide/en/elasticsearch/reference/master/index-modules-slowlog.html#_identifying_search_slow_log_origin[identifying search slow log origin] as well as https://www.elastic.co/guide/en/elasticsearch/reference/master/tasks.html#_identifying_running_tasks[identifying running tasks].
+
+The `X-Opaque-Id` should be configured in each request, for doing that you can use the `opaqueId` option, as you can see in the following example. +
+The resulting header will be `{ 'X-Opaque-Id': 'my-search' }`.
+
+[source,js]
+----
+const { Client } = require('@elastic/elasticsearch')
+const client = new Client({
+  node: 'http://localhost:9200'
+})
+
+client.search({
+  index: 'my-index',
+  body: { foo: 'bar' }
+}, {
+  opaqueId: 'my-search'
+}, (err, result) => {
+  if (err) console.log(err)
+})
+----
+
+Sometimes it may be useful to prefix all the `X-Opaque-Id` headers with a specific string, in case you need to identify a specific client or server. For doing this, the client offers a top-level configuration option: `opaqueIdPrefix`. +
+In the following example, the resulting header will be `{ 'X-Opaque-Id': 'proxy-client::my-search' }`.
+[source,js]
+----
+const { Client } = require('@elastic/elasticsearch')
+const client = new Client({
+  node: 'http://localhost:9200',
+  opaqueIdPrefix: 'proxy-client::'
+})
+
+client.search({
+  index: 'my-index',
+  body: { foo: 'bar' }
+}, {
+  opaqueId: 'my-search'
+}, (err, result) => {
+  if (err) console.log(err)
+})
+----