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.

After apply this patch, we will get the following metrics for logging:
 a) Events of Log::error():
    proxy.process.log.event_log_error_ok
    proxy.process.log.event_log_error_skip
    proxy.process.log.event_log_error_aggr
    proxy.process.log.event_log_error_full
    proxy.process.log.event_log_error_fail

 b) Events for Log::access():
    proxy.process.log.event_log_access_ok
    proxy.process.log.event_log_access_skip
    proxy.process.log.event_log_access_aggr
    proxy.process.log.event_log_access_full
    proxy.process.log.event_log_access_fail

 c) Logging Data to/from network/disk:
    proxy.process.log.num_sent_to_network
    proxy.process.log.num_received_from_network
    proxy.process.log.num_flush_to_disk
    proxy.process.log.bytes_sent_to_network
    proxy.process.log.bytes_received_from_network
    proxy.process.log.bytes_flush_to_disk
    proxy.process.log.bytes_written_to_disk

 d) Mix:
    proxy.process.log.log_files_open
    proxy.process.log.log_files_space_used

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

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