convert from measurements -> performance metrics

text/0003-browser-js-built-in-metrics.md

@@ -9,11 +9,13 @@
 
 This RFC details expanding list of built-in performance metrics for the browser JavaScript SDK, with additional data the Browser SDK captures. It propose adding two metrics that are already captured by the SDK: `connection.rtt` and `connection.downlink`, and three brand new metrics that are not yet captured, `device.memory`, `hardware.concurrency`, and `long_task.count`.
 
+Note: `performance metrics` (external name) ==== `measurements` (internal name) for the purpose of this document.
+
 # Background
 
-The Sentry product now supports the ability to set [Performance Metrics in the product](https://docs.sentry.io/product/sentry-basics/metrics), via attaching numeric tags to transaction data. Internally, we refer to these numeric tags as `measurements`. Some of these performance metrics are considered "built-in", and are automatically sent from certain SDKs. These built-in measurements are defined in an [explicit allowlist in Sentry's Relay config](https://github.com/getsentry/sentry/blob/dddb995d6f33527cc5fd2b6c6d484b29bb02253d/src/sentry/relay/config/__init__.py#L407-L428), and are [defined in Relay themselves as well](https://github.com/getsentry/relay/blob/4f3e224d5eeea8922fe42163552e8f20db674e86/relay-server/src/metrics_extraction/transactions.rs#L270-L276). 
+The Sentry product now supports the ability to set [Performance Metrics in the product](https://docs.sentry.io/product/sentry-basics/metrics), via attaching numeric tags to transaction data. Internally, we refer to these numeric tags as `measurements`. Some of these performance metrics are considered "built-in", and are automatically sent from certain SDKs. These built-in performance metrics are defined in an [explicit allowlist in Sentry's Relay config](https://github.com/getsentry/sentry/blob/dddb995d6f33527cc5fd2b6c6d484b29bb02253d/src/sentry/relay/config/__init__.py#L407-L428), and are [defined in Relay themselves as well](https://github.com/getsentry/relay/blob/4f3e224d5eeea8922fe42163552e8f20db674e86/relay-server/src/metrics_extraction/transactions.rs#L270-L276). 
 
-The Browser JavaScript SDKs currently has [seven built-in measurements](https://docs.sentry.io/platforms/javascript/performance/instrumentation/performance-metrics/), `fp`, `fcp`, `lcp`, `fid`, `cls`, `ttfb`, and `ttfb.requesttime`. In addition to built-in measurements, the product supports sending arbitrary custom performance metrics on transactions. For example in JavaScript:
+The Browser JavaScript SDKs currently has [seven built-in performance metrics](https://docs.sentry.io/platforms/javascript/performance/instrumentation/performance-metrics/), `fp`, `fcp`, `lcp`, `fid`, `cls`, `ttfb`, and `ttfb.requesttime`. In addition to built-in performance metrics, the product supports sending arbitrary custom performance metrics on transactions. For example in JavaScript:
 
 ```ts
 const transaction = Sentry.getCurrentHub().getScope().getTransaction();
@@ -22,17 +24,17 @@ const transaction = Sentry.getCurrentHub().getScope().getTransaction();
 transaction.setMeasurement('localStorageRead', 4);
 ```
 
-In the product, we display the built-in performance metrics and custom performance metrics in different sections of the event details. In addition, there is a limit to the number of custom performance metrics that can be set on a transaction (TODO on what exactly this is).
+In the product, we display the built-in performance metrics and custom performance metrics in different sections of the event details. In addition, transactions have a [limit of 5 custom performance metrics that they can send](https://github.com/getsentry/sentry/blob/dddb995d6f33527cc5fd2b6c6d484b29bb02253d/src/sentry/relay/config/__init__.py#L430-L431).
 
 # Proposals
 
 ## Existing Data
 
-Aside from the seven built-in measurements the JavaScript SDKs set, the JavaScript SDK also sets [`connection.rtt` and `connection.downlink` as performance metrics](https://github.com/getsentry/sentry-javascript/blob/74db5275d8d5a28cfb18c5723575ea04c5ed5f02/packages/tracing/src/browser/metrics/index.ts#L396-L402). Since these are not in the built-in allow list in Relay/Sentry, **they are considered custom performance metrics, and take away from the custom performance metric quota that exists on transactions**.
+Aside from the seven built-in performance metrics the JavaScript SDKs set, the JavaScript SDK also sets [`connection.rtt` and `connection.downlink` as performance metrics](https://github.com/getsentry/sentry-javascript/blob/74db5275d8d5a28cfb18c5723575ea04c5ed5f02/packages/tracing/src/browser/metrics/index.ts#L396-L402). Since these are not in the built-in allow list in Relay/Sentry, **they are considered custom performance metrics, and take away from the custom performance metric quota that exists on transactions**.
 
 `connection.rtt` is the [the estimated effective round-trip time of the current connection, rounded to the nearest multiple of 25 milliseconds](https://developer.mozilla.org/en-US/docs/Web/API/NetworkInformation/rtt). `connection.downlink` is the [effective bandwidth estimate in megabits per second, rounded to the nearest multiple of 25 kilobits per seconds](https://developer.mozilla.org/en-US/docs/Web/API/NetworkInformation/downlink). These were originally added to the SDK [Oct 2020](https://github.com/getsentry/sentry-javascript/pull/2966) to help grab info about network connectivity information.
 
-Either we choose to promote these two values to built-in measurements, or we remove them entirely if we feel like they are not high value.
+Either we choose to promote these two values to built-in performance metrics, or we remove them entirely if we feel like they are not high value.
 
 ## New Data