“=” is an exact match in Prometheus.
To do a match on multiple values it would be: “=~” e.g.:
ping_rtt_mean_ms{environment=~"value1|value2|value3"}
or with a variable:
ping_rtt_mean_ms{environment=~"$environment"}
And with variables you may want to select a custom “All” string of .* so it matches on any value. See here for more info: