new(
cfg,logger) ->client,err
Create a buffering ostium client
cfg.socket is the ostium unix socket path; nil disables the client
entirely (every method becomes a cheap no-op). cfg.app is the
source identity on every wire line. Optional keys with defaults:
flush_interval 10, tick 1, reconnect_cooldown 5, max_series 4096,
max_commands 1024. Sub-second tick and cooldown values are allowed,
which tests rely on.