GraphQL: Unlock goroutine throttle to match batch capacity (#20146)

We were throttling the amount of concurrent resolvers
at a given time. The idea behind this was to avoid overloading
the database with too many requests.

However, with the introduction of dataloaders, this limitation
actually becomes a bottleneck because all DB calls are actually
batched, so we are unnecessarily throttling the amount of
items that can be processed in a single batch.

The only caveat with this is that now all resolvers
need to backed by dataloaders, or otherwise not be queried
as part of a loop.

In a subsequent PR, we will be removing channel stats
from under channel to be a top-level object to be returned
for a given channel.

```release-note
NONE
```

api4/graphql.go

@@ -14,6 +14,7 @@ import (
 	gqlerrors "github.com/graph-gophers/graphql-go/errors"
 	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/shared/mlog"
+	"github.com/mattermost/mattermost-server/v6/web"
 )
 
 type graphQLInput struct {
@@ -22,6 +23,19 @@ type graphQLInput struct {
 	Variables     map[string]interface{} `json:"variables"`
 }
 
+// Unique type to hold our context.
+type ctxKey int
+
+const (
+	webCtx            ctxKey = 0
+	rolesLoaderCtx    ctxKey = 1
+	channelsLoaderCtx ctxKey = 2
+	teamsLoaderCtx    ctxKey = 3
+	usersLoaderCtx    ctxKey = 4
+)
+
+const loaderBatchCapacity = web.PerPageMaximum + 100
+
 //go:embed schema.graphqls
 var schemaRaw string
 
@@ -35,7 +49,8 @@ func (api *API) InitGraphQL() error {
 	opts := []graphql.SchemaOpt{
 		graphql.UseFieldResolvers(),
 		graphql.Logger(mlog.NewGraphQLLogger(api.srv.Log)),
-		graphql.MaxParallelism(5),
+		graphql.MaxParallelism(loaderBatchCapacity), // This is dangerous if the query
+		// uses any non-dataloader backed object. So we need to be a bit careful here.
 	}
 
 	if isProd() {
@@ -58,18 +73,6 @@ func (api *API) InitGraphQL() error {
 	return nil
 }
 
-// Unique type to hold our context.
-type ctxKey int
-
-const (
-	webCtx            ctxKey = 0
-	rolesLoaderCtx    ctxKey = 1
-	channelsLoaderCtx ctxKey = 2
-	teamsLoaderCtx    ctxKey = 3
-)
-
-const loaderBatchCapacity = 200
-
 func (api *API) graphQL(c *Context, w http.ResponseWriter, r *http.Request) {
 	var response *graphql.Response
 	defer func() {
@@ -111,6 +114,9 @@ func (api *API) graphQL(c *Context, w http.ResponseWriter, r *http.Request) {
 	teamsLoader := dataloader.NewBatchedLoader(graphQLTeamsLoader, dataloader.WithBatchCapacity(loaderBatchCapacity))
 	reqCtx = context.WithValue(reqCtx, teamsLoaderCtx, teamsLoader)
 
+	usersLoader := dataloader.NewBatchedLoader(graphQLUsersLoader, dataloader.WithBatchCapacity(loaderBatchCapacity))
+	reqCtx = context.WithValue(reqCtx, usersLoaderCtx, usersLoader)
+
 	response = api.schema.Exec(reqCtx,
 		params.Query,
 		params.OperationName,