Keep deployment-name flag instead of director label

[gberche-orange]

I'd suggest to keep the cf_exporter's deployment-name flag (the bosh deployment name) which gets exported in the director label (rather than the apparent renaming into director label).

Rationales:

• some credhub server are shared among multiple bosh directors
• keep consistency with cf-exporter to allow mapping from credhub server to associated bosh deployment (e.g. if the credhub exporter includes health metrics in the future to be able to browse to the associated bosh deployment)

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Original extract from https://github.com/bosh-prometheus/cf_exporter

Flag / Environment Variable	Required	Default	Description
cf.deployment-name CF_EXPORTER_CF_DEPLOYMENT_NAME	Yes		Cloud Foundry Deployment Name to be reported as a metric label
metrics.environment CF_EXPORTER_METRICS_ENVIRONMENT	Yes		Environment label to be attached to metrics

Metric	Description	Labels
metrics.namespace_application_info	Labeled Cloud Foundry Application information with a constant 1 value	environment, deployment, application_id, application_name, buildpack, organization_id, organization_name, space_id, space_name, stack_id, state

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Current version in orange-cloudfoundry/credhub_exporter

Flag / Environment Variable	Required	Default	Description
metrics.director-name CREDHUB_EXPORTER_METRICS_DIRECTOR	Yes	bosh	Director label to be attached to metrics
metrics.environment CREDHUB_EXPORTER_METRICS_ENVIRONMENT	Yes		Environment label to be attached to metrics
--	--	--	--

Metric	Description	Labels
metrics.namespace_credential_created_at	Unix timestamp of the creation of the last version of given credential	director, environment, id, name, path

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Suggested wording improvement:

Flag / Environment Variable	Required	Default	Description
metrics.deployment-name CF_EXPORTER_CF_DEPLOYMENT_NAME	Yes		Credhub Bosh Deployment Name to be reported as the deployment metric label
metrics.environment CREDHUB_EXPORTER_METRICS_ENVIRONMENT	Yes		Credhub environment label to be attached to metrics

Metric	Description	Labels
metrics.namespace_credential_created_at	Unix timestamp of the creation of the last version of given credential	deployment, environment, id, name, path