Adding the jetstream stream first_seq value

[rayjanoka]

Hey!

I was in need of the first_seq value to understand how far behind in the sequence the oldest message in the stream is. This is when using WorkQueuePolicy or InterestPolicy to delete messages from the stream as they are consumed. I think this metric is usually called data freshness. The nats server already had this value available to grab so it was pretty simple.

nats_stream_first_seq{account="$G",cluster="nats",domain="",is_meta_leader="true",is_stream_leader="true",meta_leader="nats-4",server_id="xxxx",server_name="nats-4",stream_leader="nats-4",stream_name="stream"} 952350

thx! 🚀

[wallyqs]

LGTM