fixes to secondary dispatch hedger

[vroldanbet]

• we observed a panic caused by tdigest. Even after vendoring Fix OOB access in Quantile() influxdata/tdigest#32, the issue kept happening. Given the library also appears unmaintained, we switched to https://github.com/caio/go-tdigest.
• we missed the cancelation of the primary if the secondary had returned a value. This is key to avoid the computational cost of running the primary if the secondary already executed
• restructure context cancelation to be finer-grained
• reduce the amount of time it waits before fire off the primary by using p95 without offset. This reduces the impact over calls that rely on the primary (e.g. secondary does not have an actual answer)
• added metrics for the computed hedge wait duration, and another one to determine the ratio of cancelled / executed primary dispatches
• added trace logging
• optimized usage of CEL expression so it's not compiled on each dispatch
• fixed panic when logging a DispatchLookupSubjectsResponse

[miparnisari]

TestCheckUsesDelayByDefaultForPrimary seems flaky: https://github.com/authzed/spicedb/actions/runs/14893361910/job/41830618214?pr=2382

ok  	github.com/authzed/spicedb/internal/dispatch/keys	1.144s
panic: panic raised

goroutine 906 [running]:
github.com/authzed/spicedb/internal/dispatch/remote.(*fakeDispatchSvc).DispatchCheck(0xc0030002d0, {0x1c0b5b0, 0xc0007b6c90}, 0x30?)
	/home/runner/work/spicedb/spicedb/internal/dispatch/remote/cluster_test.go:57 +0x[46](https://github.com/authzed/spicedb/actions/runs/14893361910/job/41830618214?pr=2382#step:4:47)7
github.com/authzed/spicedb/pkg/proto/dispatch/v1._DispatchService_DispatchCheck_Handler({0x18a8160, 0xc0030002d0}, {0x1c0b5b0, 0xc0007b6c90}, 0xc002f00300, 0x0)
	/home/runner/work/spicedb/spicedb/pkg/proto/dispatch/v1/dispatch_grpc.pb.go:174 +0x28f
google.golang.org/grpc.(*Server).processUnaryRPC(0xc0003d7400, {0x1c0b5b0, 0xc0007b6c00}, 0xc0007993e0, 0xc0009df800, 0x25bf220, 0x0)
	/home/runner/go/pkg/mod/google.golang.org/grpc@v1.72.0/server.go:1405 +0x1bc3
google.golang.org/grpc.(*Server).handleStream(0xc0003d7400, {0x1c0b9b0, 0xc00038bd40}, 0xc0007993e0)
	/home/runner/go/pkg/mod/google.golang.org/grpc@v1.72.0/server.go:1815 +0x1373
google.golang.org/grpc.(*Server).serveStreams.func2.1()
	/home/runner/go/pkg/mod/google.golang.org/grpc@v1.72.0/server.go:1035 +0x159
created by google.golang.org/grpc.(*Server).serveStreams.func2 in goroutine 989
	/home/runner/go/pkg/mod/google.golang.org/grpc@v1.72.0/server.go:1046 +0x215
FAIL	github.com/authzed/spicedb/internal/dispatch/remote	0.933s
FAIL