Add basic metrics in storage service (#872)

1. Add basic metrics for data operation in storage service
2. Add a storage integrity test tool in storage-perf.

close #287

Now we support following metrics: get_bound, bound_stats, vertex_props, edge_props, add_vertex, add_edge, del_vertex, update_vertex, update_edge.

We can get qps or latency from these metrics (add suffix), for example: vertex_props_qps or vertex_props_latency.

You can get a single metrics like this:
curl -G "http://127.0.0.1:50005/get_stats?stats=vertex_props_qps.rate.60

If you want get multiple metrics like this:
curl -G "http://127.0.0.1:50005/get_stats?stats=vertex_props_qps.rate.60,vertex_props_latency.avg.60,vertex_props_latency.p99.60&returnjson"
We can get qps, avg latency and p99 latency in a minutue.

You can get all metrics like this:
curl -G "http://127.0.0.1:50005/get_stats?stats"

The storage integrity tool is used for verifying data integrity in storage. It only works for single replica for now. You can use it like this:
./storage_integrity --meta_server_addrs=192.168.8.5:60001 --first_vertex_id=100 --width=1000 --height=1000

meta_server_addrs is the meta server addresss.
first_vertex_id is the first vertex id which we inserts.
width and height is the width and height of the big matrix.

src/executor/test/CMakeLists.txt

@@ -29,6 +29,7 @@ set(GRAPH_TEST_LIBS
     $<TARGET_OBJECTS:meta_gflags_man_obj>
     $<TARGET_OBJECTS:gflags_man_obj>
     $<TARGET_OBJECTS:ws_common_obj>
+    $<TARGET_OBJECTS:stats_obj>
 )
 
 nebula_add_library(