Allow users to select the cloud metadata providers (elastic#13812)

* Allow users to select the cloud metadata providers

We introduce a new setting 'providers' to the add_cloud_metadata
processor.

By now all the implementation for metadata providers requires developers
to mark a provider as 'local'. The alibaba and tencent providers are not
marked as local by now.

If the 'providers' setting is not used, then no all providers marked as
'local' are applied. This is a breaking change, because alibaba and
tencent providers will not be enabled anymore by default.

If the providers setting is used, only the selected providers will be
used.

CHANGELOG.next.asciidoc

@@ -17,6 +17,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fixed a crash under Windows when fetching processes information. {pull}12833[12833]
 - Update to Golang 1.12.7. {pull}12931[12931]
 - Remove `in_cluster` configuration parameter for Kuberentes, now in-cluster configuration is used only if no other kubeconfig is specified {pull}13051[13051]
+- Disable Alibaba Cloud and Tencent Cloud metadata providers by default. {pull}13812[12812]
 
 *Auditbeat*
 
@@ -257,6 +258,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Add autodetection mode for add_docker_metadata and enable it by default in included configuration files{pull}13374[13374]
 - Added `monitoring.cluster_uuid` setting to associate Beat data with specified ES cluster in Stack Monitoring UI. {pull}13182[13182]
 - Add autodetection mode for add_kubernetes_metadata and enable it by default in included configuration files. {pull}13473[13473]
+- Add `providers` setting to `add_cloud_metadata` processor. {pull}13812[13812]
 
 *Auditbeat*
 

libbeat/docs/processors-using.asciidoc

@@ -505,8 +505,8 @@ not:
 === Add cloud metadata
 
 The `add_cloud_metadata` processor enriches each event with instance metadata
-from the machine's hosting provider. At startup it will detect the hosting
-provider and cache the instance metadata.
+from the machine's hosting provider. At startup it will query a list of hosting
+providers and cache the instance metadata.
 
 The following cloud providers are supported:
 
@@ -518,6 +518,10 @@ The following cloud providers are supported:
 - Azure Virtual Machine
 - Openstack Nova
 
+The Alibaba Cloud and Tencent cloud providers are disabled by default, because
+they require to access a remote host. The `providers` setting allows users to
+select a list of default providers to query.
+
 The simple configuration below enables the processor.
 
 [source,yaml]
@@ -526,7 +530,7 @@ processors:
 - add_cloud_metadata: ~
 -------------------------------------------------------------------------------
 
-The `add_cloud_metadata` processor has two optional configuration settings.
+The `add_cloud_metadata` processor has three optional configuration settings.
 The first one is `timeout` which specifies the maximum amount of time to wait
 for a successful response when detecting the hosting provider. The default
 timeout value is `3s`.
@@ -535,7 +539,20 @@ If a timeout occurs then no instance metadata will be added to the events. This
 makes it possible to enable this processor for all your deployments (in the
 cloud or on-premise).
 
-The second optional configuration setting is `overwrite`. When `overwrite` is
+The second optional setting is `providers`. The `providers` settings accepts a
+list of cloud provider names to be used. If `providers` is not configured, then
+all providers that do not access a remote endpoint are enabled by default.
+
+List of names the `providers` setting supports:
+- "alibaba", or "ecs" for the Alibaba Cloud provider (disabled by default).
+- "azure" for Azure Virtual Machine (enabled by default).
+- "digitalocean" for Digital Ocean (enabled by default).
+- "aws", or "ec2" for Amazon Web Services (enabled by default).
+- "gcp" for Google Copmute Enging (enabled by default).
+- "openstack", or "nova" for Openstack Nova (enabled by default).
+- "tencent", or "qcloud" for Tencent Cloud (disabled by default).
+
+The third optional configuration setting is `overwrite`. When `overwrite` is
 `true`, `add_cloud_metadata` overwrites existing `cloud.*` fields (`false` by
 default).