feat: introduce MR metrics

[arththebird]

Description of your changes

Consume crossplane/crossplane-runtime#683 and configure metrics for Workspaces.

Heavily inspired by crossplane-contrib/provider-kubernetes#224 and crossplane-contrib/provider-upjet-aws#1281

Fixes #271

I have:

• Run make reviewable to ensure this PR is ready for review.

How has this code been tested

1. Deploy the provider locally using make local-deploy
2. Get the metrics from the provider pod (curl -s http://localhost:8080/metrics | grep crossplane)

# HELP crossplane_managed_resource_exists The number of managed resources that exist
# TYPE crossplane_managed_resource_exists gauge
crossplane_managed_resource_exists{gvk="tf.upbound.io/v1beta1, Kind=Workspace"} 1
# HELP crossplane_managed_resource_ready The number of managed resources in Ready=True state
# TYPE crossplane_managed_resource_ready gauge
crossplane_managed_resource_ready{gvk="tf.upbound.io/v1beta1, Kind=Workspace"} 1
# HELP crossplane_managed_resource_synced The number of managed resources in Synced=True state
# TYPE crossplane_managed_resource_synced gauge
crossplane_managed_resource_synced{gvk="tf.upbound.io/v1beta1, Kind=Workspace"} 1

[bobh66]

LGTM - thanks!