fix pgbouncer_stats_totals_server_assignments_total metric

[AndrewJackson2020]

It seems that the server_assignment_count metric is misspelled and thus it is not showing up/being collected. Are there any tests that can be added for this as well? Also updated the test TestQueryShowStats.

Also not sure why DCO is failing, I definitely signed the commit. Feel free to take the code in this PR and raise it in a different PR if needed.

 > psql postgresql://postgres@localhost:6432/pgbouncer -x -c 'SHOW stats_totals;'
-[ RECORD 1 ]-----------+----------
database                | pgbouncer
server_assignment_count | 0
xact_count              | 8
query_count             | 8
bytes_received          | 0
bytes_sent              | 0
xact_time               | 0
query_time              | 0
wait_time               | 0
client_parse_count      | 0
server_parse_count      | 0
bind_count              | 0

[SuperQ]

This needs a DCO sign-off. You can use git commit -s --amend to add it.

[SuperQ]

Commit sha: aab36b1, Author: Andrew Jackson, Committer: Andrew Jackson; Expected "Andrew Jackson [email protected]", but got "Andrew Jackson [email protected]".

[AndrewJackson2020]

Okay, the DCO is fixed now.