[6.2.0] profile: add profile_finish_ts (bazelbuild#18129)

"date" field in Bazel json profile is a confusing field.

It uses Java Date.toString() output which includes local timezone
that could be hard to parse.

The name is misleading since users could mistook it for profile
start time, while it actually means the finish/end time, when
Bazel calls the writer to serialize profiling data to disk.

Add "profile_finish_ts" which has a clearer name and more consistent
value with the time unit being used in the rest of the JSON
profile(microseconds).

We shall deprecate the "date" field in a separate patch in a major
release.

site/en/rules/performance.md

@@ -278,6 +278,7 @@ Example:
   "otherData": {
     "build_id": "101bff9a-7243-4c1a-8503-9dc6ae4c3b05",
     "date": "Tue Jun 16 08:30:21 CEST 2020",
+    "profile_finish_ts": "1677666095162000",
     "output_base": "/usr/local/google/_bazel_johndoe/573d4be77eaa72b91a3dfaa497bf8cd0"
   },
   "traceEvents": [

src/main/java/com/google/devtools/build/lib/profiler/Profiler.java

@@ -39,6 +39,7 @@
 import java.lang.management.ManagementFactory;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
+import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Date;
@@ -1086,13 +1087,15 @@ public void run() {
                 // The buffer size of 262144 is chosen at random.
                 new OutputStreamWriter(
                     new BufferedOutputStream(outStream, 262144), StandardCharsets.UTF_8))) {
+          var finishDate = Instant.now();
           writer.beginObject();
           writer.name("otherData");
           writer.beginObject();
           writer.name("bazel_version").value(BlazeVersionInfo.instance().getReleaseName());
           writer.name("build_id").value(buildID.toString());
           writer.name("output_base").value(outputBase);
-          writer.name("date").value(new Date().toString());
+          writer.name("date").value(finishDate.toString());
+          writer.name("profile_finish_ts").value(finishDate.getEpochSecond() * 1000);
           writer.endObject();
           writer.name("traceEvents");
           writer.beginArray();