add scroll depth as stats API v2 metric

Author: RobertJoonas

docs/stats-api.md

@@ -84,7 +84,8 @@ Valid metrics are:
 | `bounce_rate` | `float` | Bounce rate percentage | |
 | `visit_duration` | `int` | Visit duration in seconds | |
 | `events` | `int` | The number of events (pageviews + custom events). When filtering by a goal, this metric corresponds to "Total Conversions" in the dashboard. | |
-| `percentage` | `float` | The percentage of visitors of total who fall into this category: Requires: dimension list | Requires non-empty `dimensions` |
+| `scroll_depth` | `int` | Page scroll depth averaged per session | Requires `event:page` filter or dimension being set |
+| `percentage` | `float` | The percentage of visitors of total who fall into this category | Requires non-empty `dimensions` |
 | `conversion_rate` | `float` | The percentage of visitors who completed the goal. | Requires non-empty `dimensions`, `event:goal` filter or dimension being set |
 | `group_conversion_rate` | `float` | The percentage of visitors who completed the goal with the same dimension. Requires: dimension list passed, an event:goal filter or event:goal dimension | Requires non-empty `dimensions`, event:goal filter or dimension being set |
 | `average_revenue` | `Revenue` or null | Average revenue per revenue goal conversion | Requires [revenue goals](docs/ecommerce-revenue-tracking.md), `event:goal` filter or dimension for a relevant revenue goal. |