Export account resource metrics and configuration

[MattiasAng]

Adds four new metrics that collects configured reserved memory and storage along with how many bytes are currently used of each.
This will allow users to monitor resource consumption based on set maximum.

I've decided to name the configured reserved metrics as max to align with configuration file rather than what the API response attribute is named (reserved).

Example response:

# HELP jetstream_account_max_memory JetStream Account Max Memory in bytes
# TYPE jetstream_account_max_memory gauge
jetstream_account_max_memory{account="test",account_id="test",cluster="",domain="",is_meta_leader="true",meta_leader="",server_id="http://localhost:8222",server_name="test"} 1.048576e+07
# HELP jetstream_account_max_storage JetStream Account Max Storage in bytes
# TYPE jetstream_account_max_storage gauge
jetstream_account_max_storage{account="test",account_id="test",cluster="",domain="",is_meta_leader="true",meta_leader="",server_id="http://localhost:8222",server_name="test"} 1.048576e+06
# HELP jetstream_account_memory_used Total number of bytes used by JetStream memory
# TYPE jetstream_account_memory_used gauge
jetstream_account_memory_used{account="test",account_id="test",cluster="",domain="",is_meta_leader="true",meta_leader="",server_id="http://localhost:8222",server_name="test"} 0
# HELP jetstream_account_storage_used Total number of bytes used by JetStream storage
# TYPE jetstream_account_storage_used gauge
jetstream_account_storage_used{account="test",account_id="test",cluster="",domain="",is_meta_leader="true",meta_leader="",server_id="http://localhost:8222",server_name="test"} 448245