Both Uses of

herald

in

A Tale of Two Cities

Mrs. Southcott had recently attained her five-and-twentieth blessed birthday, of whom a prophetic private in the Life Guards had heralded the sublime appearance by announcing that arrangements were made for the swallowing up of London and Westminster.†
Chpt 1.1 *

Heralded by a courier in advance, and by the cracking of his postilions' whips, which twined snake-like about their heads in the evening air, as if he came attended by the Furies, Monsieur the Marquis drew up in his travelling carriage at the posting-house gate.†
Chpt 2.8

Definitions:

(1) (herald as in: heralds the coming of...)
a sign that something will happen -- especially something important and good

or:

to announce or welcome -- especially with enthusiasm
(2) (herald as in: announced by herald & trumpet)
a person who announces important news -- especially a king's representative
(3) (meaning too rare to warrant focus)
More rarely, herald can also refer to an official whose specialty is heraldry (the study of coats of arms -- symbols once used to signify the right to bear arms).

Historically a herald was a person who made announcements at jousting matches, or (much more rarely) any prestigious assistant in a dignified setting such as a courthouse or royal palace.