Log errors related to gathering stats and updating metrics instead of exiting the process

[busma13]

Updates the teraslice-exporter to catch, log, and count errors when calling terasliceStats.update() and updateTerasliceMetrcis() instead of exiting. Any http request that does not return a 200 status code is considered an error, so network issues will cause the exporter to exit.

Additional output added to /metrics endpoint:

# HELP teraslice_exporter_errors Number of errors encountered by teraslice exporter.
# TYPE teraslice_exporter_errors gauge
teraslice_exporter_errors{error_type="update_stats_errors"} 0
teraslice_exporter_errors{error_type="update_metrics_errors"} 0

Example of logged errors:

{"name":"teraslice_exporter","terasliceUrl":"http://localhost:5678/","hostname":"Peters-MacBook-Pro.local","pid":56281,"level":50,"msg":"Error encountered getting terasliceStats: Error: Error getting http://localhost:5678/v1/cluster/state: HTTPError: Response code 500 (Internal Server Error)","time":"2024-01-02T22:36:08.957Z","v":0}
{"name":"teraslice_exporter","terasliceUrl":"http://localhost:5678/","hostname":"Peters-MacBook-Pro.local","pid":56281,"level":50,"msg":"Error encountered getting terasliceStats: Error: Error getting http://localhost:5678/v1/cluster/controllers: HTTPError: Response code 500 (Internal Server Error)","time":"2024-01-02T22:36:19.031Z","v":0}

Fixes: #12