Onset parent for RPC-based tracers, user span for cache subrequests

[mar-cf]

No description provided.

[ask-bonk]

The in-tree CacheClientImpl drops the new userSpanParent before it starts the cache-service subrequest. As a result, cache.match(), put(), and delete() still begin the downstream request without a user trace trigger, despite Cache::getHttpClient() supplying one. Forward it alongside the internal parent.

    kj::Own<kj::HttpClient> getDefault(CacheClient::SubrequestMetadata metadata) override {
      return kj::heap<CacheHttpClientImpl>(*cacheService, cacheNamespaceHeader, kj::none,
          kj::mv(metadata.cfBlobJson), kj::mv(metadata.parentSpan),
          kj::mv(metadata.userSpanParent));
    }

    kj::Own<kj::HttpClient> getNamespace(
        kj::StringPtr cacheName, CacheClient::SubrequestMetadata metadata) override {
      auto encodedName = kj::encodeUriComponent(cacheName);
      return kj::heap<CacheHttpClientImpl>(*cacheService, cacheNamespaceHeader, kj::mv(encodedName),
          kj::mv(metadata.cfBlobJson), kj::mv(metadata.parentSpan),
          kj::mv(metadata.userSpanParent));
    }

   private:
    kj::Own<IoChannelFactory::SubrequestChannel> cacheService;
    kj::HttpHeaderId cacheNamespaceHeader;
  };

  class CacheHttpClientImpl final: public kj::HttpClient {
   public:
    CacheHttpClientImpl(IoChannelFactory::SubrequestChannel& parent,
        kj::HttpHeaderId cacheNamespaceHeader,
        kj::Maybe<kj::String> cacheName,
        kj::Maybe<kj::String> cfBlobJson,
        SpanParent parentSpan,
        SpanParent userSpanParent)
        : client(asHttpClient(parent.startRequest({.cfBlobJson = kj::mv(cfBlobJson),
              .parentSpan = kj::mv(parentSpan),
              .userSpanParent = kj::mv(userSpanParent)}))),

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

Updates tracer onset parenting and cache subrequest user-span propagation.

1. [P1] src/workerd/server/server.c++:4282 drops userSpanParent, so cache subrequests remain detached from user traces. Suggestion posted.

Time for a pun! This cache trace has a parent problem: it was never properly forwarded.

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