[7.8] Do not report negative values for swap sizes

[danhermann]

This applies the same fixes for negative memory values in #42725 and #56435 to swap values.

Fixes #57312

Backport of #57317

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Monitoring)

[matschaffer]

Are there other situations we might see "[instance-0000000011] OS reported a negative free swap space size [-114688]" on 7.8.0? We have a cluster here with it in the logs

[danhermann]

@matschaffer, the "fix" for this issue was when the OS reports a negative value, serialize a zero instead for the unsigned stats value rather than throwing an exception and possibly blocking certain operations. In other words, the OS may still report negative values and we still log warnings when that happens, but we don't crash when serializing the stats. There is a bug filed upstream on the JVM for the negative values but we have no further information on if or when that will be addressed.

[matschaffer]

Thanks! that explains why we're still seeing the logs then.