TS-2145: Add metrics for log collation

Without accurate stats, we don't known whether the log is lost.
This patch is used to verify, such as:

1) At log client side, how many logs genterated from HttpSM, how many
   logs sent to LogServer.

2) At log server side, how many logs received from LogClient, how many
   logs written to LogServers's disk.

BTW: Remove LOG_XXX stats macro according Leif's suggestion.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>