Zookeeper-3735: fix the bad format of RATE_LOGGER

[jonomorris]

The message written to the Rate-logger was modified to conform with the way the Rate-logger composes messages it writes to the underlying logger.

RateLogger updates:

1. Added Javadoc and Unit-tests.
2. Updated code to omit logging the value when none is provided, and to prevent a previously logged value from being re-written after a flush.

[maoling]

I also found other two nits: we need to log zxid with Hex. Could you plz also fix them in passing:)
RATE_LOGGER.rateLimitLog("Digests are not matching. Value is Zxid.", String.valueOf(zxid));

[maoling]

Why do you need this change? Time interval has elapsed, reset count?

[jonomorris]

If the time interval has elapsed call flush writing the existing value and current message count then save the message with new value provided as the first message received after the flush with count 1.

This change makes it consistent with an explicit flush, where the flush method is called on the RateLogger object, which writes the current message along with exiting value and current message count.

[maoling]

• Haha, not very persuasive for me:).
• And for test, when time interval has elapsed and at the same time, flush() is called explicitly, the tests will become complicated.

e.g:
A Permutation and Combination of these three lines
currentTime.set(DEFAULT_LOGGING_INTERVAL + 1L);
rateLogger.rateLimitLog("test message one.", "value-four");
rateLogger.flush();

[jonomorris]

Hey @maoling, yes sorry there's a bit going on in that last unit test. I can simplify it to just test the flush after setting the elapsed time:

currentTime.set(DEFAULT_LOGGING_INTERVAL + 1L);

I guess I just wanted to demonstrate that how the message and value are retained after the message is flushed when the logging-interval is exceeded.

Did you want this bit of code in RateLogger reverted?

[maoling]

@jono-morris
Take it easy, let's wait here for other people's review and comment

[maoling]

this test may be flaky, you can look at other example how to mock timing (e.g.: CreateContainerTest#testMaxNeverUsedInterval)

[jonomorris]

Updated RateLogger to get current time from a method with package access that is overridden during unit testing. This allows the time to be explicitly set similar to the way you have suggested.

[jonomorris]

Also updated RateLogger calls in reportDigestMismatch and reportDigestMismatch methods to use
"0x" + Long.toHexString(zxid)

[jonomorris]

Not required