Add flag to print more condensed build stats

The new flag is called --experimental_stats_summary. I do not consider
this flag an incompatible change, since we don't guarantee the exact
text output on the command-line. The intent is to switch to the new
summary style and then retire the flag.

The new stats look like this:
INFO: 5,000 processes: 1,000 remote, 1,000 local, 1,000 worker, 2,000 cached
INFO: Total action wall time 100.34s
INFO: Critical path 5.22s (setup 1.34s, wall time 3.89s)
INFO: Elapsed time 8.89s (loading 2.55s, execution 6.34s)

The breakdown in the last two lines follows the idea that we want to
distinguish between useful work and 'overhead':
- For the critical path breakdown, the wall time is the sum of all wall
  times of all actions on the critical path, whereas 'setup' refers to
  everything other than subprocess execution wall time.
- For the elapsed time breakdown, the execution time is the time of the
  execution phase (which in turn is determined by the critical path),
  whereas 'loading' refers to everything other than execution.

Terminology is hard and we're already using these terms to denote
different things. Suggestions welcome.

PiperOrigin-RevId: 222231747

src/main/java/com/google/devtools/build/lib/exec/ExecutionOptions.java

diff --git a/src/main/java/com/google/devtools/build/lib/exec/ExecutionOptions.java b/src/main/java/com/google/devtools/build/lib/exec/ExecutionOptions.java
index 0f9e2c2..34aeffb 100644
--- a/src/main/java/com/google/devtools/build/lib/exec/ExecutionOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/exec/ExecutionOptions.java
@@ -300,6 +300,15 @@
   public boolean enableCriticalPathProfiling;
 
   @Option(
+      name = "experimental_stats_summary",
+      documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
+      effectTags = {OptionEffectTag.TERMINAL_OUTPUT},
+      defaultValue = "false",
+      help = "Enable a modernized summary of the build stats."
+  )
+  public boolean statsSummary;
+
+  @Option(
     name = "experimental_execution_log_file",
     defaultValue = "",
     category = "verbosity",