Metadata API should tell if the data set is fixed size, or not #2336

Example:

• Visit per hour, Visit duration, Pages per visit are tables of fixed sizes
• countries, browsers, and many others are open reports that can be between 0 and N rows

It would be useful to have such information in each metadata output, so that metadata users could decide better how to display the reports, in particular #1721

Added in [http://piwik.org/docs/analytics-api/metadata/ Analytics Metadata API] documentation: 'constantRowsCount' - if set to 1, it means the report always has the same number of rows. For example, "Visit per hour" always has 24 hours, "Visits per number of pages" has 10 rows. This attribute is set only when there is a 'dimension' attribute. If this attribute is not set, it means that the returned data set could have 0 row, 1 row, or N rows.

(In [4510]) Fixes #2336
'constantRowsCount' - if set to 1, it means the report always has the same number of rows. For example, "Visit per hour" always has 24 hours, "Visits per number of pages" has 10 rows. This attribute is set only when there is a 'dimension' attribute. If this attribute is not set, it means that the returned data set could have 0 row, 1 row, or N rows.