wt-status: add trace2 data for sparse-checkout percentage

When sparse-checkout is enabled, add the sparse-checkout percentage to
the Trace2 data stream.  This number was already computed and printed
on the console in the "You are in a sparse checkout..." message.  It
would be helpful to log it too for performance monitoring.

Signed-off-by: Jeff Hostetler <jeffhostetler@github.com>

Author: jeffhostetler

wt-status.c

@@ -2571,6 +2571,36 @@ void wt_status_print(struct wt_status *s)
 			   s->untracked.nr);
 	trace2_data_intmax("status", s->repo, "count/ignored", s->ignored.nr);
 
+	switch (s->state.sparse_checkout_percentage) {
+	case SPARSE_CHECKOUT_DISABLED:
+		break;
+	case SPARSE_CHECKOUT_SPARSE_INDEX:
+		/*
+		 * Log just the observed size of the sparse-index.
+		 *
+		 * When sparse-index is enabled we can have
+		 * sparse-directory entries in addition to individual
+		 * sparse-file entries, so we don't know the complete
+		 * size of the index.  And we do not want to force
+		 * expand it just to emit some telemetry data.  So we
+		 * cannot report a percentage for the space savings.
+		 *
+		 * It is possible that if the telemetry data is
+		 * aggregated, someone will have a good estimate for
+		 * the size of a fully populated index and can compute
+		 * a percentage after the fact.
+		 */
+		trace2_data_intmax("status", s->repo,
+				   "sparse-index/size",
+				   s->repo->index->cache_nr);
+		break;
+	default:
+		trace2_data_intmax("status", s->repo,
+				   "sparse-checkout/percentage",
+				   s->state.sparse_checkout_percentage);
+		break;
+	}
+
 	trace2_region_enter("status", "print", s->repo);
 
 	switch (s->status_format) {