Extend the LogSink interface to be able to pass microseconds

[asekretenko]

The goal of this PR is to solve the problem of passing microseconds into custom LogSink implementations that was described in #307.

I am proposing a slightly different approach, however: instead of modifying the existing send() signature I am adding the second one.
This has an advantage of not breaking all the third-party LogSink implementations immediately (including ours) ... and, unfortunately, all the disadvantages associated with this approach.

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[asekretenko]

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[jpeach]

@asekretenko would it make sense for the usecs parameter to be unsigned?

[asekretenko]

@asekretenko would it make sense for the usecs parameter to be unsigned?

Probably it makes sense.
I cannot remember any way to obtain a negative value for (x - static_cast<some_interger_type>(x)) * 1e-6 for an IEEE754-compliant double x if x>0. And you?

[asekretenko]

@asekretenko would it make sense for the usecs parameter to be unsigned?

On the other hand, on some crazy (or "slightly buggy") platform we might get a negative value here for some corner case. Seeing ".-000001" in the microseconds would help to understand what the hell is going on much better than seeing seemingly normal values in a strange order. Been there, done that (albeit with another logging system).

Probably it would be better to follow advice from Google's style guide

Do not use an unsigned type merely to assert that a variable is non-negative.

and leave microseconds signed.

[asekretenko]

@shinh @sergiud May I ask you to have a look at this PR?