fix: limit calls to GetWorkspaceAgentByID in agentapi (#23015)

We currently call GetWorkspaceAgentByID millions of times at scale
unnecessarily. This PR embeds immutable fields into the relevant
services instead of fetching for them every time.

resolves https://github.com/coder/scaletest/issues/84

Confirmed with a 10k scaletest that this changeset takes the query from
10M+ queries down to 39k

coderd/agentapi/metadata.go

@@ -5,18 +5,18 @@ import (
 	"fmt"
 	"time"
 
+	"github.com/google/uuid"
 	"golang.org/x/xerrors"
 
 	"cdr.dev/slog/v3"
 	agentproto "github.com/coder/coder/v2/agent/proto"
 	"github.com/coder/coder/v2/coderd/agentapi/metadatabatcher"
 	"github.com/coder/coder/v2/coderd/database"
-	"github.com/coder/coder/v2/coderd/database/dbauthz"
 	"github.com/coder/coder/v2/coderd/database/dbtime"
 )
 
 type MetadataAPI struct {
-	AgentFn   func(context.Context) (database.WorkspaceAgent, error)
+	AgentID   uuid.UUID
 	Workspace *CachedWorkspaceFields
 	Database  database.Store
 	Log       slog.Logger
@@ -45,29 +45,11 @@ func (a *MetadataAPI) BatchUpdateMetadata(ctx context.Context, req *agentproto.B
 		maxErrorLen = maxValueLen
 	)
 
-	// Inject RBAC object into context for dbauthz fast path, avoid having to
-	// call GetWorkspaceByAgentID on every metadata update.
-	var err error
-	rbacCtx := ctx
-	if dbws, ok := a.Workspace.AsWorkspaceIdentity(); ok {
-		rbacCtx, err = dbauthz.WithWorkspaceRBAC(ctx, dbws.RBACObject())
-		if err != nil {
-			// Don't error level log here, will exit the function. We want to fall back to GetWorkspaceByAgentID.
-			//nolint:gocritic
-			a.Log.Debug(ctx, "Cached workspace was present but RBAC object was invalid", slog.F("err", err))
-		}
-	}
-
-	workspaceAgent, err := a.AgentFn(rbacCtx)
-	if err != nil {
-		return nil, err
-	}
-
 	var (
 		collectedAt = a.now()
 		allKeysLen  = 0
 		dbUpdate    = database.UpdateWorkspaceAgentMetadataParams{
-			WorkspaceAgentID: workspaceAgent.ID,
+			WorkspaceAgentID: a.AgentID,
 			// These need to be `make(x, 0, len(req.Metadata))` instead of
 			// `make(x, len(req.Metadata))` because we may not insert all
 			// metadata if the keys are large.
@@ -121,7 +103,7 @@ func (a *MetadataAPI) BatchUpdateMetadata(ctx context.Context, req *agentproto.B
 	}
 
 	// Use batcher to batch metadata updates.
-	err = a.Batcher.Add(workspaceAgent.ID, dbUpdate.Key, dbUpdate.Value, dbUpdate.Error, dbUpdate.CollectedAt)
+	err := a.Batcher.Add(a.AgentID, dbUpdate.Key, dbUpdate.Value, dbUpdate.Error, dbUpdate.CollectedAt)
 	if err != nil {
 		return nil, xerrors.Errorf("add metadata to batcher: %w", err)
 	}