feat: implement boundary usage tracker and telemetry collection (#21716)

Implements telemetry for boundary usage tracking across all Coder
replicas and reports them via telemetry.

Changes:
- Implement Tracker with Track(), FlushToDB(), and StartFlushLoop() methods
- Add telemetry integration via collectBoundaryUsageSummary()
- Use telemetry lock to ensure only one replica collects per period

The tracker accumulates unique workspaces, unique users, and request
counts (allowed/denied) in memory, then flushes to the database
periodically. During telemetry collection, stats are aggregated across
all replicas and reset for the next period.

coderd/workspaceagentsrpc.go

@@ -148,6 +148,7 @@ func (api *API) workspaceAgentRPC(rw http.ResponseWriter, r *http.Request) {
 		PublishWorkspaceUpdateFn:          api.publishWorkspaceUpdate,
 		PublishWorkspaceAgentLogsUpdateFn: api.publishWorkspaceAgentLogsUpdate,
 		NetworkTelemetryHandler:           api.NetworkTelemetryBatcher.Handler,
+		BoundaryUsageTracker:              api.BoundaryUsageTracker,
 
 		AccessURL:                 api.AccessURL,
 		AppHostname:               api.AppHostname,