Explainer for PA per-participant metrics

FLEDGE_extended_PA_reporting.md

@@ -214,6 +214,7 @@ Where `signalBucket` and `signalValue` is a dictionary which consists of:
       * generateBid() hitting timeout
       * The auction was aborted (i.e. calling endAdAuction())
       * an auction that never rendered the ad
+  * Some additional values described [separately](#per-participant-base-values).
 * optional `offset` and `scale` that allow the reporter to manipulate the browser signal to customize the buckets and values they will receive in reports:
   * `scale` will be multiplied by the browser provided value. Scale is applied before `offset` is added. Default value is 1.0.
   * `offset` will be added to the browser provided value (allows you to shift buckets, etc). Default value is 0.
@@ -255,6 +256,82 @@ The `reserved.always` event-type is a special value that indicates a report shou
 matter whether a bid wins the auction or not. The browser will always trigger reporting for the
 `always` contributions.
 
+## Per-participant metrics
+
+More recent versions of Chrome offer some additional features, focused on measuring resource
+usage and whole-auction metrics rather than those specific to a particular function execution
+or the winning bid.
+
+First, the new `reserved.once` event-type is a special value that, for each sub-auction, selects a
+random invocation of `generateBid()` and of `scoreAd()` independently, and reports private
+aggregation contributions with that event only from those executions. (In case of a multi-seller
+auction, the top-level auction will have a single `scoreAd()` invocation selected as well).
+
+The event may not be used in `reportWin()` or `reportResult()`; since those already run at most
+once per auction level, `reserved.always` may be used instead.
+
+This feature is intended for reporting overall per-participant metrics only once rather than for
+every interest group participating. A number of new `baseValues` representing such values are
+available and described below, but it can also be useful with per-IG metrics which are not expected to
+vary much like `signals-fetch-time`, to sample them.
+
+While usage of `reserved.once` will be ignored by older versions, newly added `baseValues` will not
+be, so the calls to `contributeToHistogramOnEvent()` should be individually wrapped in `try/catch`.
+That is also encouraged in general since `contributeToHistogramOnEvent()` is specified to throw
+on permission policy violations.
+
+Users using this are strongly encouraged to report their metrics during the beginning of their
+scripts, since if the script hits a per script time out before asking to report them nothing will
+get sent, which can result in inaccuracy, especially for `percent-scripts-timeout`.
+
+### Per-participant base values.
+
+The newly added base values are as following:
+* `participating-ig-count`: number of interest groups that got a chance to participate in this
+  (sub)auction, i.e. they had registered ads, and if no timeouts were to hit, their `generateBid()`
+  would have been run.
+* `average-code-fetch-time`: average time of all code fetches (including JavaScript and WASM) used
+  for all invocations of particular function type for given (sub)auction.
+* `percent-scripts-timeout`: percentage of script executions of this kind that hit the per-script
+  timeout.
+* `percent-igs-cumulative-timeout`: percentage of interest groups from this buyer that did not get
+  to participate in this (sub)auction due to the per-buyer cumulative timeout
+  (`participating-ig-count` is the quotient here).
+* `cumulative-buyer-time`: total time spent for buyer's computation, in milliseconds; this is what
+  would normally be  compared against the per-buyer cumulative timeout. If the timeout is not hit,
+  the value is capped at the per-buyer cumulative timeout, if it's hit, the value will be the
+  timeout + 1.
+* `percent-regular-ig-count-quota-used`,`percent-negative-ig-count-quota-used`,
+  `percent-ig-storage-quota-used`: percentage of the database quote used by the buyer for
+  regular interest group count, negative targeting interest group count, and overall byte usage
+  respectively. This is capped at 110 since the quotas may not be enforced immediately (and actual
+  usage in that case may be bigger than 110%).
+
+Note that these metrics are measured only for some kinds of worklet executions — some are
+only relevant for bidders, and get 0 in the seller functions. In case of reporting functions,
+they sometimes repeat what was available in the corresponding `generateBid()` or `scoreAd()`,
+and sometimes get their own measurement. This is shown below:
+
+| `baseValue` name | In `generateBid() ` | In `reportWin()` | In `scoreAd()` | In `reportResult()` |
+| ---------------- | ------------------- | ---------------- | -------------- | ------------------- |
+| `participating-ig-count`  | Measured | From `generateBid()`  | 0 | 0 |
+| `average-code-fetch-time` | Measured | Measured | Measured | Measured |
+| `percent-scripts-timeout` | Measured | Measured | Measured | Measured |
+| `percent-igs-cumulative-timeout` | Measured | From `generateBid()` | 0 | 0 |
+| `cumulative-buyer-time` | Measured | From `generateBid()` | 0 | 0 |
+| `percent-regular-ig-count-quota-used` | Measured | From `generateBid()` | 0 | 0 |
+| `percent-negative-ig-count-quota-used` | Measured | From `generateBid()` | 0 | 0 |
+| `percent-ig-storage-quota-used` | Measured | From `generateBid()` | 0 | 0 |
+
+For example, `percent-scripts-timeout` in `generateBid()` is the portion of executions of
+`generateBid()` in that sub-auction that timed out, while `percent-scripts-timeout` in
+`reportWin()` is either 0 or 100 dependent on whether the reporting function's execution timed out
+or not (assuming reporting is done early enough to happen if it did); while
+`percent-igs-cumulative-timeout` will be the same value in both.
+
+Similarly, `percent-scripts-timeout` makes sense for seller functions like `scoreAd()`, but
+`percent-igs-cumulative-timeout` doesn't, so it just evaluates to 0.
+
 ## Reporting Per-Buyer Latency and Statistics to the Seller
 
 The seller may want to collect aggregate statistics on latency and bids for their auctions.