To allow for different API keys per namespace, we can't memoize the
metrics URL like we did before. In preparation for this, this patch
moves the call to must_metrics_url_from_env to evaluate just in time.

In preparation of namespaced data, replace the existing HashSet with a
HashMap, which will hold a key per namespace. For now, place all data
under the "default" namespace. Then, loop over each namespace in the
out variable when sending metrics to AppSignal, and send all matrics
for one namespace as one request.

Update the must_metrics_url_from_env function to take a namespace as
an argument, in preparation for having multiple API keys. For now,
always return the URL with the default API key.

To allow for selecting a different key for a different namespace, this
patch adds another environment variable that has a list namespaces and
API keys. If one matches, api_key_for_namespace returns the associated
key instead of the default.

Instead of hardcoding the namespace to "default", use the pod's actual
namespace when looking up the metric URL.

In the deployment, get the value from the namespace-api-key-map secret
and configure it as the APPSIGNAL_NAMESPACE_API_KEY_MAP.

To allow for testing the namespaced API keys, suffix the version
number so an image can be pushed without having to do a proper release.

This is a branch separate from the rest of the integration, so any
releases shouldn't be tagged as latest.