Add docs about ping body limits and changelog entry

Also...
* Correct data-review link for discarded_exceeding_pings_size
* Add changelog entry about ping tagging for iOS (forgot on previous PR)

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 [Full changelog](https://github.com/mozilla/glean/compare/v31.6.0...main)
 
+* General
+  * Limit ping request body size to 1MB. ([#1098]((https://github.com/mozilla/glean/pull/1098)))
+* iOS
+  * Implement ping tagging (i.e. the `X-Source-Tags` header) through custom URL ([#1100](https://github.com/mozilla/glean/pull/1100)).
 * C#
   * Add support for Labeled Strings and Labeled Booleans.
   * Add support for the Counter metric type and Labeled Counter.

docs/user/pings/index.md

@@ -80,8 +80,6 @@ See [Using the Experiments API](../experiments-api.md) on how to record experime
 
 The pings that the Glean SDK generates are submitted to the Mozilla servers at specific paths, in order to provide additional metadata without the need to unpack the ping payload.
 
-> **Note**: To keep resource usage in check, the Glean SDK allows only up to 10 ping submissions every 60 seconds. There are no exposed methods to change these rate limiting defaults yet, follow [Bug 1647630](https://bugzilla.mozilla.org/show_bug.cgi?id=1647630) for updates.
-
 A typical submission URL looks like
 
   `"<server-address>/submit/<application-id>/<doc-type>/<glean-schema-version>/<document-id>"`
@@ -94,6 +92,13 @@ where:
 - `<glean-schema-version>`: the version of the Glean ping schema;
 - `<document-id>`: a unique identifier for this ping.
 
+### Limitations
+
+To keep resource usage in check, the Glean SDK enforces some limitations on the pings it will upload and in the amount of uploads allowed per a given time interval.
+
+- **Rate limiting**: only up to 10 ping submissions every 60 seconds are allowed. There are no exposed methods to change these rate limiting defaults yet, follow [Bug 1647630](https://bugzilla.mozilla.org/show_bug.cgi?id=1647630) for updates.
+- **Request body size limiting**: the body of a ping request may have up to 1MB. Pings that exceed this size are discarded and don't get uploaded. Size and number of discarded pings are recorded on the internal Glean metric [`glean.upload.discarded_exceeding_pings_size`](../collected-metrics/metrics.md#metrics-1).
+
 ### Submitted headers
 A pre-defined set of headers is additionally sent along with the submitted ping:
 

glean-core/metrics.yaml

@@ -383,7 +383,7 @@ glean.upload:
     bugs:
       - https://bugzilla.mozilla.org/show_bug.cgi?id=1597761
     data_reviews:
-      - https://bugzilla.mozilla.org/show_bug.cgi?id=1597761#c4
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1597761#c10
     data_sensitivity:
       - technical
     notification_emails: