Create initial exporter

[bendrucker]

Creates the initial version of the exporter

https://takescoop.atlassian.net/browse/PLATFORM-2413

[bendrucker]

At the moment, this is a simplified program that fetches agent pools once and exports the metrics to stdout.

Example output from a local run:

[
	{
		"Name": "agents{service.name=unknown_service:terraform-cloud-metrics-exporter,telemetry.sdk.language=go,telemetry.sdk.name=opentelemetry,telemetry.sdk.version=1.7.0,instrumentation.name=terraform_cloud,pool=default,status=idle}",
		"Last": 2,
		"Timestamp": "2022-05-12T16:07:56.779199-07:00"
	},
	{
		"Name": "agents{service.name=unknown_service:terraform-cloud-metrics-exporter,telemetry.sdk.language=go,telemetry.sdk.name=opentelemetry,telemetry.sdk.version=1.7.0,instrumentation.name=terraform_cloud,pool=default,status=exited}",
		"Last": 18,
		"Timestamp": "2022-05-12T16:07:56.779202-07:00"
	}
]

A working initial version would have:

• A polling loop that fetches the agents, records the metrics, and then sleeps a configurable amount of time
• The ability to direct metrics output in OTLP format to a Datadog agent (https://github.com/open-telemetry/opentelemetry-go/tree/main/exporters/otlp/otlpmetric/otlpmetricgrpc) rather than just stdout

With those two things in place as well as some general refactoring/restructuring (e.g. adding CLI flags, splitting into packages) I should be able to move on to deploy. Probably another 1-2 days work here.

[bendrucker]

Re permissions needed to fetch this data, I've observed:

• Owners can manage (modify) agent resources, as documented
• Organization tokens can list agent pools and agents
• Other teams cannot grant agent access. By default, a team has no access to agent APIs. I tried granting all available organization access to a test team. I found its team token is able to list the agent pools, but when listing the agents within a pool, the response is an empty list. Guessing "manage workspaces" confers permission to list agent pools, but not the agents themselves.

An org token seems to be our best near term choice. While still fairly privileged to perform write operations, it's certainly less privileged than an owners token:

https://www.terraform.io/cloud-docs/users-teams-organizations/api-tokens

[ryanwholey]

Hope you don't mind a few go-specific questions! The logic for counting agents makes sense to me.