Add "timestamp_key", prefer log message when merging

[bendrucker]

• Adds timestamp_key property, which allows the user to customize the default key of timestamp (+ tests)
• Changes merge order of json_merge so that the log message wins over the record data in cases of conflict (+ tests)
• Updates bundler to 2 (unrelated, required to run tests)

Notes

• Switching the symbol key ({ :timestamp => 123 }) to a string { 'timestamp' => 123 } is important. JSON.dump({ :foo => 'bar', 'foo' => 'qux' }) will return {"foo:"bar","foo":"qux"} so we need key types to match.
• The change in merge order is the most invasive but it seems sensible to prioritize user data over record data in the event of conflict. I doubt anyone has run into this yet and I had to write a failing test to understand how it works.

Motivation

Scoop uses a standard logging library across our apps that emits newline-delimited JSON. It includes timestamps as {"time": 123}. A dev noticed that log lines from Kubernetes contained both timestamp and time. Because we use json_merge, both properties appear at the same level. We'd like to configure fluentd to render a timestamp as time to any log line missing it (including non-JSON) but not overwrite an existing time property in JSON lines.

[frankreno]

LGTM, thanks! I'll be cutting a new release tomorrow or Friday. Will you be submitting a PR to the Helm repo once this gets released?

edit: HAH, Im mixing my repos! I am going to cut a new FluentD K8S image, when I do, ill upgrade this plugin version to ruby gems and update it in the k8s plugin

[bendrucker]

I'll send a PR to the k8s repo adding the ability to configure this via env and then add it to the chart as well

[frankreno]

@bendrucker no worries on that, already started as I also want to upgrade the fluentD image...working on that but hit a couple snags I am working thru

[bendrucker]

Sounds good!

[frankreno]

@bendrucker - this is now available in the k8s fluentD plugin.