Add number of running flushes & compactions to --stats_per_interval output (#9726)

Summary:
This is for https://github.com/facebook/rocksdb/issues/9709 and add two lines to the end of DB Stats
for num-running-compactions and num-running-flushes.

For example ...

** DB Stats **
Uptime(secs): 6.0 total, 1.0 interval
Cumulative writes: 915K writes, 915K keys, 915K commit groups, 1.0 writes per commit group, ingest: 0.11 GB, 18.95 MB/s
Cumulative WAL: 915K writes, 0 syncs, 915000.00 writes per sync, written: 0.11 GB, 18.95 MB/s
Cumulative stall: 00:00:0.000 H:M:S, 0.0 percent
Interval writes: 133K writes, 133K keys, 133K commit groups, 1.0 writes per commit group, ingest: 16.62 MB, 16.53 MB/s
Interval WAL: 133K writes, 0 syncs, 133000.00 writes per sync, written: 0.02 GB, 16.53 MB/s
Interval stall: 00:00:0.000 H:M:S, 0.0 percent
num-running-compactions: 0
num-running-flushes: 0

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9726

Reviewed By: jay-zhuang

Differential Revision: D35066759

Pulled By: mdcallag

fbshipit-source-id: c161fadd3c15c5aa715a820dab6bfedb46dc099b

tools/db_bench_tool.cc

@@ -2211,7 +2211,13 @@ class Stats {
               }
             } else if (db) {
               if (db->GetProperty("rocksdb.stats", &stats)) {
-                fprintf(stderr, "%s\n", stats.c_str());
+                fprintf(stderr, "%s", stats.c_str());
+              }
+              if (db->GetProperty("rocksdb.num-running-compactions", &stats)) {
+                fprintf(stderr, "num-running-compactions: %s\n", stats.c_str());
+              }
+              if (db->GetProperty("rocksdb.num-running-flushes", &stats)) {
+                fprintf(stderr, "num-running-flushes: %s\n\n", stats.c_str());
               }
               if (FLAGS_show_table_properties) {
                 for (int level = 0; level < FLAGS_num_levels; ++level) {