This has been deprecated [since 2011][0]

[0]: https://github.com/rubygems/rubygems/blob/1aebd7ddd69c9a38aa9daa3aa89f396d59c9e2a4/History.txt#L3296-L3297

Use the rubocop generated configuration to ignore any major changes that
can't be auto-corrected.

Ruby 2.4 is now end-of-life. As a result of this, the `google-protobuf`
gem is no longer releasing artifacts compatible with this version.
(protocolbuffers/protobuf#7453)

In CI we encounter this error from `bundle install`:
```
google-protobuf-3.12.0-x86_64-linux requires ruby version >= 2.5, which is
incompatible with the current version, ruby 2.4.6p354
```

Therefore remove Ruby 2.4.6 as a tested version in Travis CI.

Additionally remove the constraint on patch versions in the Travis
config, so that we'll use the latest patch version available for each
release branch.

Augment the existing operations with Prometheus metrics in order to
provide observability around the operations that the plugin is
performing.

Introduce a new metrics helper to prevent attempting to register the
same metric more than once in a multi-threaded or multi-instance
context.

As per the README updates, this can be used to compress a number of
input records into a single Pub/Sub message, therefore saving on costs.