feat(kit/feature): add feature flag package

[gavincabbage]

This PR adds basic feature flag capabilities to InfluxDB. It allows users to create, consume and manipulate feature flags to exercise experimental feature before they become default behavior. The majority of code in this PR has been deployed internally for the past month and is merely open sourced by this change.

The best description of the functionality provided by this PR is in the included doc.go file:

Package feature provides feature flagging capabilities for InfluxDB servers. This document describes this package and how it is used to control experimental features in influxd.

Flags are configured in flags.yml at the top of this repository. Running make flags generates Go code based on this configuration to programmatically test flag values in a given request context. Boolean flags are the most common case, but integers, floats and strings are supported for more complicated experiments.

The Flagger interface is the crux of this package.
It computes a map of feature flag values for a given request context. The default implementation always returns the flag default configured in flags.yml. The override implementation allows an operator to override feature flag defaults at startup. Changing these overrides requires a restart.

In influxd, a Flagger instance is provided to a Handler middleware
configured to intercept all API requests and annotate their request context with a map of feature flags.

A flag can opt in to be exposed externally in flags.yml. If exposed, this flag will be included in the response from the new /api/v2/flags endpoint. This allows the UI and other API clients to control their behavior according to the flag in addition to the server itself.

A concrete example to illustrate the above:

I have a feature called "My Feature" that will involve turning on new code in both the UI and the server.

First, I add an entry to flags.yml.

- name: My Feature
  description: My feature is awesome
  key: myFeature
  default: false
  expose: true
  contact: My Name

My flag type is inferred to be boolean by my defaulf of false when I run make flags and the feature package now includes func MyFeature() BoolFlag.

I use this to control my backend code with

if feature.MyFeature.Enabled(ctx) {
  // new code...
} else {
  // new code...
}

and the /api/v2/flags response provides the same information to the frontend.

{
  "myFeature": false
}

While false by default, I can turn on my experimental feature by starting my server with a flag override.

env INFLUXD_FEATURE_FLAGS="{\"flag1\":\value1\",\"key2\":\"value2\"}" influxd

influxd --feature-flags flag1=value1,flag2=value2

• CHANGELOG.md updated with a link to the PR (not the Issue)
• Well-formatted commit messages
• Rebased/mergeable
• Tests pass
• http/swagger.yml updated (if modified Go structs or API)
• Documentation updated or issue created (Document feature flags docs-v2#976)

[glinton]

Hey there @gavincabbage, i see you're into force pushing. Since you've requested reviews, please refrain from force-pushing until at least all the reviews are complete as it makes it difficult to track your recent work and wastes more time.

[desa]

LGTM. Just a couple nits/comments. Overall really clean.

[desa]

Will d being nil cause a panic here?

[gavincabbage]

I tested with this flag both set and not and didn't run into any panics. It's just used as a default, and in launcher.go we don't use this map if its nil or empty anyway.

[gavincabbage]

Thanks for taking a look @desa!

[glinton]

Nice work!

[brettbuddin]

Wonderful work here, @gavincabbage.