SQL Server Input Plugin - Add availability group configuration and state metrics

## Feature Request

To monitor the health / configuration of SQL Server availability group. It would be good to add TSQL some queries where results would be collected by the SQL Server Input Plugin.

### Proposal:

#### Underlying Windows Failover Cluster info

`SELECT
member_name,
member_type_desc AS member_type,
member_state_desc AS member_state,
number_of_quorum_votes
FROM sys.dm_hadr_cluster_members;`

#### AG level info => Recovery health + synchronization state

`SELECT
g.name as ag_name,
rgs.primary_replica,
rgs.primary_recovery_health_desc AS [primary_recovery_health],
rgs.synchronization_health_desc AS [synchronization_health]
FROM sys.dm_hadr_availability_group_states as rgs
JOIN sys.availability_groups AS g ON rgs.group_id = g.group_id;`

#### AG replica level info => synchronization state + operational state + recovery health

`SELECT
g.name as ag_name,
LEFT(r.replica_server_name, CHARINDEX('\', r.replica_server_name) - 1) AS replica_cluster_name,
r.replica_server_name,
rs.is_local,
role_desc AS [role],
rs.operational_state_desc AS [operational_state],
rs.connected_state_desc AS [connected_state],
rs.recovery_health_desc AS [recovery_health],
rs.synchronization_health_desc AS [synchronization_health]
FROM sys.dm_hadr_availability_replica_states AS rs
JOIN sys.availability_replicas AS r
ON rs.replica_id = r.replica_id
JOIN sys.availability_groups AS g
ON g.group_id = r.group_id`

#### AG database level info => synchronization state + operational state + database state recovery health

`SELECT
g.name as ag_name,
LEFT(r.replica_server_name, CHARINDEX('\', r.replica_server_name) - 1) AS replica_cluster_name,
r.replica_server_name,
DB_NAME(drs.database_id) AS [database_name],
drs.is_local,
drs.is_primary_replica,
drs.synchronization_health_desc AS [synchronization_health],
drs.synchronization_state_desc AS [synchronization_state],
drs.database_state_desc AS [database_state],
drs.is_suspended,
drs.suspend_reason_desc AS [suspend_reason],
drs.secondary_lag_seconds
FROM sys.dm_hadr_database_replica_states AS drs
JOIN sys.availability_replicas AS r
ON r.replica_id = drs.replica_id
JOIN sys.availability_groups AS g
ON g.group_id = drs.group_id
ORDER BY g.name, drs.is_primary_replica DESC, drs.database_id `

### Current behavior:

Currently, no metrics about AG config , state or health

### Desired behavior:

Having these metrics added

### Use case:

Currently, availability group topology and configuration / status can't be included via telegraf and other tools or extra work is required. Having these metrics available may help to get a better picture of availability groups and their states coupling to existing performance counters available with query_version = 2

Start by locating the SQL Server Input Plugin and reviewing how its existing query_version = 2 metrics are collected and named. Use the four proposed availability-group queries as the scope, then verify that cluster, AG, replica, and database state metrics are collected and exposed consistently.

Written by the indexing model from the issue text.