Improve javadoc for ExtraInfoDescriptor classes.

See merge request !4

Parse and expose fields from the overload-general line from the server descriptor.

See merge request !5

OverloadRatelimitsVersion and OverloadFdExhaustedVersion were defined as int
but parsed as long.

Also include test for OverloadRatelimitsVersion.

Remove javadoc warnings at build time and improve
overall code documentation.

* Checks that the overload keys are seen at most once
* Exposes the version fields from each line
* Exposes the rate and burst limits from overload-ratelimit
* Checks that the line has all the fields present

relay-search#40005

Update readme with development section with quick instructions to start developing on
metrics-lib

#40006

Fixes #40005.

Fixes: #40004

This test only ensures that the new fields do
not cause the parser to crash, it does not look
at actually parsing those fields.

Fixes: #40003

Implements #40002.

When parsing a large file with many descriptors we would repeatedly
search the remaining file for the sequence "newline + keyword + space"
and then "newline + keyword + newline" to find the start of the next
descriptor. However, if the keyword is always followed by newline, the
first search would always fail.

The optimization here is to search once whether the keyword is
followed by space or newline and avoid unnecessary searches when going
through the file.

In the long term we should use a better parser. But in the short term
this optimization will have a major impact on performance, in
particular with regard to concatenated microdescriptors.

Implements the library part of tpo/metrics/statistics#40002.