[FEATURE] Change the "master" nomenclature

[tlfeng]

Is your feature request related to a problem?
OpenSearch repository is going to replace the terminology "master"with "cluster manager".
issue: opensearch-project/OpenSearch#472 (comment), with the plan for its terminology replacement.

The existing OpenSearch REST APIs and settings will be deprecated in version 2.0, and eventually be removed in at least version 3.0. Please prepare for the nomenclature change in advance.

What solution would you like?
Replace the terminology "master" with "cluster manager".

1. Inform the user about the nomenclature change in advance in the documentation.
2. Replace all the internal usages with "master" word in the code base.

The impact to the OpenSearch user of the nomenclature change:

• REST APIs and settings that contain "master" word will be deprecated in version 2.0. They are still able to be used, but using deprecated APIs and settings will result in a deprecation warning logged in file.
• Alternative usages with "cluster_manager" word is preferred to be used, which are provided for every deprecated usages.

For detailed code change, please see the issue opensearch-project/OpenSearch#1549.

List of REST API and setting changes:
For all the non-inclusive REST APIs and settings list, please see the replacement plan of master terminology for OpenSearch repository: opensearch-project/OpenSearch#472 (comment)
Setting names

1 cluster.initial_master_nodes (static)
New name: cluster.initial_cluster_manager_nodes

2 cluster.no_master_block (dynamic)
New name: cluster.no_cluster_manager_block

3 cluster.service.slow_master_task_logging_threshold (dynamic)
New name: cluster.service.slow_cluster_manager_task_logging_threshold

Expected behavior (same as the deprecated OpenDistro settings of plugins):

• If the new setting is configured, value of the old setting will be ignored.
• If the new setting is not configured, value of the old setting will be valid.

Setting value

1 node.roles: [ master ]
New name of the master role : cluster_manager

• A new node role will be added, aims to replace the existing master role. New node created by OpenSearch 2.0 will have cluster_manager role and without master role by default, but they have got the same functionality.
• New node will have "cluster_manager", "data", and "ingest" roles by default. Which means the default value of setting "node.roles" will be ["cluster_manager","data","ingest"], instead of ["master","data","ingest"]
• master role and cluster_manager role can not be assigned together to a node.
• See the link for detailed changes in responses of REST APIs Add a new node role 'cluster_manager' as the alternative for 'master' role and deprecate 'master' role OpenSearch#2424.

REST API endpoint
1 GET _cat/master
New name: GET _cat/cluster_manager

• Using the old path _cat/master and new path _cat/cluster_manager will get the same response.

REST API path parameter
1 "node filters" used in some cluster-level APIs. The APIs are usually in the format: GET /_nodes/<node_id>, where <node_id> can be set as _master or master:true or master:false to filter the master nodes.

• The filter names are based on the node roles, since a new node role is added, cluster_manager can also be used to filter nodes to be shown in the API response. Besides, master and cluster_manager have got the same filtering effect, and they will filter the node with either role.

REST API request parameter name
1 master_timeout=
New name: cluster_manager_timeout=

• A new optional parameter cluster_manager_timeout will be added, and it has the same effect with the existing master_timeout. An exception will be thrown if values are assigned to the both parameters.
• The parameter is widely used in REST APIs

REST API request parameter value
1 metric=master_node
New name: metric=cluster_manager_node

• Use either master_node or cluster_manager_node can get the same metrics.
• So far, the metric parameter is used in GET /_cluster/state/<metrics>/<target> and POST /_cluster/reroute APIs

REST API response field
1 discovered_master in the response of GET _cluster/health
New name: discovered_cluster_manager

• a new field will be added to GET _cluster/health API: discovered_cluster_manager, though the "old" field discovered_master has not been added to the document yet.

2 discovered_master in the response of GET _cat/health

• the header discovered_master of the table in the response of GET _cat/health API will be replaced by discovered_cluster_manager (breaking change)
• If user wants to get output with the old column header name master, please use h parameter to filter the column to be shown explicitly, such as GET _cat/health?v&h=master

3 master in the response of GET _cat/nodes
master -> cluster_manager

• the header master of the table in the response of GET _cat/nodes API will be replaced by cluster_manager (breaking change)
• If user wants to get output with the old column header name master, please use h parameter to filter the column to be shown explicitly, such as GET _cat/nodes?v&h=version,master

What alternatives have you considered?
none.

Do you have any additional context?
none.

[Naarcha-AWS]

Thanks @tlfeng! We'll get this in before the upcoming preview.

[cwillum]

Reopening to change the cluster.initial_master_nodes setting name to cluster.initial_cluster_manager_nodes in the sample docker compose file on the Docker image page.

[cwillum]

Boxing this back up post merge of #1151.