ref(rate limits): Add concurrent headers (#4931)

* ref(rate limits): Add concurrent headers

Author: ceorourke

src/api/ratelimits.mdx

@@ -5,9 +5,15 @@ sidebar_order: 3
 
 Sentry rate limits every API request made to prevent abuse and resource overuse. The limit is applied to each unique combination of caller and endpoint.
 
-The rate limit follows a fixed window approach. Requests are counted into time buckets, determined by the window size. If the number of requests from a caller exceeds the number of allowed requests within a window, then that request will be rejected.
 
-While there is a default rate limit of **40 requests per second**, each endpoint can have their own maximum number of requests and window size. You can track your rate limit usage by looking at special headers in the response.
+We restrict both how frequently a request is made (requests per second rate limit) and how many concurrent requests a caller can make (concurrent rate limit).
+
+The requests per second rate limit follows a fixed window approach. Requests are counted into time buckets, determined by the window size. If the number of requests from a caller exceeds the number of allowed requests within a window, then that request will be rejected. While there is a default rate limit of **40 requests per second**, each endpoint can have their own maximum number of requests and window size.
+
+Meanwhile, the concurrent rate limiter will reject requests if the caller has too many requests in progress at the same time.
+
+You can track your rate limit usage by looking at special headers in the response.
+
 
 ## Headers
 
@@ -19,6 +25,10 @@ Every API request response includes the following headers:
   - The number of requests this caller has left on this endpoint within the current window
 - `X-Sentry-Rate-Limit-Reset`
   - The time when the next rate limit window begins and the count resets, measured in UTC seconds from epoch
+- `X-Sentry-Rate-Limit-ConcurrentLimit`
+  - The maximum number of concurrent requests allowed within the window
+- `X-Sentry-Rate-Limit-ConcurrentRemaining`
+  - The number of concurrent requests this caller has left on this endpoint within the current window
 
 ## Additional Information