AP_Logger: warn on high dropped log message rate

[andyp1per]

It's easy to miss this

[IamPete1]

I have seen logs that would just result in sending this message on every check at 1hz. I think it has to be rate limited down. Typically logging rates done change that much during flight, maybe just one warning per opend log is enough.

But do agree that it is often missed, it's a common bug report on filter review that the results look odd when really there is just no data to go on.

[andyp1per]

maybe just one warning per opend log is enough.

Tricky because there is usually a burst at startup which then settles down

[IamPete1]

maybe just one warning per opend log is enough.

Tricky because there is usually a burst at startup which then settles down

@peterbarker has fixed that now, or at least on master I get 0 dropped messages. As you say we regularly got a few at log start before that.