fix(sync): write to tail metric in newTail

[tschuyebuhl]

Overview

the chain head metric celestia_lightnode_hdr_store_head_height_gauge is being written to for both tails and heads, causing the metric to report two heights

while the tail metric celestia_lightnode_hdr_store_tail_height_gauge stays flat at 0:

this is caused by the error in newTail method, this PR aims to fix that

[tschuyebuhl]

after deploying this, the tail metric went up, and the head metric is stable:

[Wondertan]

Good catch. Thank you.

[tschuyebuhl]

thanks for getting this in! and sorry for the wrong PR title