feat(measurements): Add new measurements section to transaction

src/docs/sdk/event-payloads/properties/measurements.mdx

@@ -0,0 +1,41 @@
+`measurements`
+
+: _Optional_. An object containing standard/custom measurements with keys signifying the name of the measurement.
+
+Standard measurement keys currently supported are from the following list taken from [here](https://github.com/getsentry/sentry/blob/a8c960a933d2ded5225841573d8fc426a482ca9c/static/app/utils/discover/fields.tsx#L654-L676).
+
+```json
+[
+  // web
+  "fp",
+  "fcp",
+  "lcp",
+  "fid",
+  "cls",
+  "ttfb",
+  "ttfb.requesttime",
+  // mobile
+  "app_start_cold",
+  "app_start_warm",
+  "frames_total",
+  "frames_slow",
+  "frames_frozen",
+  // react native
+  "stall_count",
+  "stall_total_time",
+  "stall_longest_time"
+]
+```
+
+Custom measurements need units to be specified. Units currently supported are durations from the list `["ns" , "ms" , "s"]` taken from [here](https://github.com/getsentry/relay/blob/1e45a8817e45408ecbdbd3f0a679a5f5e4885290/relay-metrics/src/protocol.rs#L14-L21). Note that in the future we will extend the available units (for instance, memory units) but we will not implement a generic user-defined unit interface.
+
+
+```json
+{
+  "measurements": {
+    "lcp": { "value": 100 },
+    "fp": { "value": 123 },
+    "my.custom.metric": { "value": 456, "unit": "ms" }
+  }
+}
+```

src/docs/sdk/event-payloads/transaction.mdx

@@ -61,3 +61,5 @@ import "./properties/status.mdx";
 ```
 
 import "./properties/spans.mdx";
+
+import "./properties/measurements.mdx";