Merge branch '1.6.x'

Merge branch '1.5.x' into 1.6.x

Merge branch '1.3.x' into 1.5.x

Add test for failed poll scenario

See micrometer-metricsgh-2549

Merge branch '1.5.x' into 1.6.x

Polish PrometheusMeterRegistryTest

Simplifies the assertion that the +Inf bucket histogram gauge appears only once in the scrape endpoint.
See micrometer-metricsgh-2485

Add original copyright to nullability annotations

The files were copied in 2017 and modified then as well as most recently in 2021.

Resolves micrometer-metricsgh-2421

Merge branch '1.5.x' into 1.6.x

Merge branch '1.3.x' into 1.5.x