Support for Ruby 2.7

[marcotc]

With Ruby 2.7 scheduled to be released December 2019, dd-trace-rb should be ready to support it on release date.

There are a few confirmed blockers affecting the latest release, 2.7.0-preview1:

Blockers

• 2.7.0-preview1 is affected by https://bugs.ruby-lang.org/issues/15891, which prevents dd-trace-rb from starting up successfully. This has been fixed in Ruby master branch, which is still unreleased.

Contrib

• grpc, as of version 1.23.0, doesn't support Ruby 2.7, failing installation with: grpc requires Ruby version >= 2.3, < 2.7.dev

[delner]

For grpc we could simply restrict the version support to < 2.7 until gRPC is updated.

[marcotc]

I created a branch to test this, after 2.7.0-preview3 has been released: https://github.com/DataDog/dd-trace-rb/tree/ci/ruby-2.7-preview3

All gems loaded properly, except for grpc and graphql: grpc has the same issue mentioned earlier, and graphql tries to load a file that has been removed in 2.7.

For everything else, tests pass.

I'll follow up again closer to release, but things look promising so far.

[marcotc]

PR is open in draft mode, #896, awaiting the official release of 2.7 Docker containers.