Support ACLP fetching entity metrics

[yec-akamai]

📝 Description

This PR introduced a new client MonitorClient, to make calls to a different host monitor-api. It utilizes PAT token generated by linode api. The endpoint for this client is fetching metrics for a list of entities. Also, updated test suite for the new client and test the fetching metrics endpoint.

✔️ How to Test

Unit test:

make test-unit TEST_ARGS="-run TestFetchEntityMetrics" 

Integration test:

make test-int TEST_ARGS="-run TestMonitorAPI_Fetch_Entity_Metrics"

[zliang-akamai]

Suggested change

	// FetchEntityMetrics returns metrics information for the individual entities within a specific service type
	func (mc *MonitorClient) FetchEntityMetrics(ctx context.Context, serviceType string, opts *EntityMetricsFetchOptions) (*EntityMetrics, error) {
	// GetEntityMetrics returns metrics information for the individual entities within a specific service type
	func (mc *MonitorClient) GetEntityMetrics(ctx context.Context, serviceType ServiceType, opts *GetEntityMetricsOptions) (*EntityMetrics, error) {

Should we change Fetch to Get to be consistent with the API doc?

And I think the type should be the alias of string ServiceType so users can use linodego.ServiceTypeDBaaS without casting it into string (string(linodego.ServiceTypeDBaaS)), and all other monitor functions may need this change as well.

[yec-akamai]

Using fetch here is because this endpoint is actually a POST. However, the prefix Get usually indicates an actual GET endpoint, which could be confusing. I've talked to the ACLP team about this situation, and they suggested fetch might be a better way to go.

I'm not sure if using a ServiceType string is better here. Because the ServiceType enum includes a list of services, however, here we only accept dbaas for now. It could be misleading that we also support other service types.

[zliang-akamai]

Okay, Fetch seems to be fine then. Maybe there can be a chance to update the doc though. But ServiceType is defined for this usage, right? Meaning we will get those services supported in the future?
Maybe checking with ACLP team for their opinion?

[yec-akamai]

The ServiceType was defined for another set of endpoints, Dashboard. It's a separate effort though. From what I've learnt there isn't a clear plan for what services will be support in the next for Metrics here.

[zliang-akamai]

Looks good and works well!

[jriddle-linode]

Looks good on my end and is working locally.