Fix LSP telemetry unique machine id

Rename dotty.tracing.machineId to dotty.trace.machineId: this matches how
it's named in package.json (and using the wrong name means that the
machine id was not persisted across sessions)

Author: smarter

vscode-dotty/src/tracer.ts

@@ -45,7 +45,7 @@ export class Tracer {
         this.maximumMessageSize = maximumMessageSize === undefined || maximumMessageSize < 0 ? 0 : maximumMessageSize | 0
 
         this.machineId = (() => {
-            const machineIdKey = 'tracing.machineId'
+            const machineIdKey = 'trace.machineId'
             function persisted(value: string): string {
                 ctx.extensionConfig.update(machineIdKey, value, vscode.ConfigurationTarget.Global)
                 return value