Add more metrics to Performance Metricset on MSSQL Module

[sayden]

In this PR we add more metrics to Performance Metricset on MSSQL Module. The ones added could be considered the last ones that will be added for the sake of completeness in this Metricset.

Fields added:

• logouts: Total number of logout operations started per second.
• logins: Total number of logins started per second. This does not include pooled connections.
• transactions: Total number of transactions in the instance. There's another metric that returns this value per database but it's omitted. We'll place it in a different Metricset.
• active_temp_tables: Number of temporary tables/table variables in use.
• connections_reset: Total number of logins started from the connection pool.
• buffer.checkpoint_pages: Indicates the number of pages flushed to disk per second by a checkpoint or other operation that require all dirty pages to be flushed.
• buffer.database_pages: Indicates the number of pages in the buffer pool with database content.
• buffer.target_pages: Ideal number of pages in the buffer pool.

Fields modified. I found that few metrics were prefixed with buffer or has some relation with it (a total of 5) so I place them in the same object:

• page_life_expectancy to buffer.page_life_expectancy
• buffer_cache_hit to buffer.cache_hit

More things included in this PR:

• New data.json generated file
• Docs updated to describe new fields.
• fields.yml modified and updated with the new structure of modified fields as well as new fields.
• Integration test updated to check values of the new fields as well as the modified ones.
• CHANGELOG entry.