prometheus.exporter.mssql: add support connect to MSSQL named instances

[Mykhailo-Roit]

Request

Microsoft allows connecting to Named instances using the syntax "Data Source=MySqlServer\\MSSQL1;" manual but Alloy does allow the using

prometheus.exporter.mssql "SQL2022_test" {
  connection_string = "sqlserver://@localhost\\SQL2022?authenticator=winsspi"  
  query_config = local.file.mssql_queries.content
}

and got error

Error: config.alloy:81:1: Failed to build component: building component: failed to parse connection string URL: parse "sqlserver://@localhost\\SQL2022?authenticator=winsspi": invalid character "\\" in host name

80 |
81 |   prometheus.exporter.mssql "SQL2022_test" {
   |  _^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
82 | |   connection_string = "sqlserver://@localhost\\SQL2022?authenticator=winsspi"
83 | |   query_config = local.file.mssql_queries.content
84 | | }
   | |_^
85 |

interrupt received

Use case

When several named instances are installed on one server, it will be very useful.

now I use workaround: allow TCP connection for each named instance and bind static TCP port

prometheus.exporter.mssql "SQL2022" {
  connection_string = "sqlserver://@localhost:1436?authenticator=winsspi"  
  query_config = local.file.mssql_queries.content
}

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