Expose quantity for Qt and add doc about this step

Author: brizental

docs/adding_a_new_metric_type.md

@@ -90,7 +90,7 @@ class StringMetric extends Metric<string, string> {
 }
 ```
 
-Oce you have your `Metric` subclass, include it in Glean.js'
+Once you have your `Metric` subclass, include it in Glean.js'
 [`METRIC_MAP`](https://github.com/mozilla/glean.js/blob/main/glean/src/core/metrics/utils.ts#L17).
 This map will be used as a template for creating metric instances from the metrics database.
 
@@ -112,6 +112,10 @@ Still, metric type classes will always have at least one recording function and
 > Make sure that, when you included your `Metric` class on the `METRIC_MAP` the property has the
 > same value as the `type` property on the corresponding `MetricType`.
 
+Once you are done implementing the `MetricType` class for your new metric type,
+make sure to manually expose it for Qt platforms by adding it to
+[the Qt entry point file](https://github.com/mozilla/glean.js/blob/main/glean/src/index/qt.ts).
+
 #### Recording functions
 
 _Functions that call Glean.js' database and store concrete values of a metric type._

glean/src/index/qt.ts

@@ -14,6 +14,7 @@ import CounterMetricType from "../core/metrics/types/counter.js";
 import DatetimeMetricType from "../core/metrics/types/datetime.js";
 import EventMetricType from "../core/metrics/types/event.js";
 import LabeledMetricType from "../core/metrics/types/labeled.js";
+import QuantityMetricType from "../core/metrics/types/quantity.js";
 import StringMetricType from "../core/metrics/types/string.js";
 import TimespanMetricType from "../core/metrics/types/timespan.js";
 import UUIDMetricType from "../core/metrics/types/uuid.js";