If I'm not practicing metta here, then I'm just flapping my proverbial gums. If I feel annoyance, then there's all the more reason to be mindful of it and make an intentional choice about how to proceed rather than blindly swirling in the downward spiral of the twelve links of dependent origination (rather than the upward spiral of Transcendental Dependent Arising).
There isn't a single time that I feel annoyance that I'm not clinging to something, most often to views. Annoyance cannot persist in the absence of clinging. Even if I find myself in opposition to someone's expressed view or to their overall style of interpreting the Dhamma, there is also the choice to recognize that every single person on this forum is seeking to eliminate suffering and experience happiness (as are all sentient beings). If I'm responding to someone (or even thinking about someone in the absence of responding) and I'm not doing it from a place of metta, then what am I really accomplishing? It's more downward spiral, which is contrary to the purpose of participating here. There's a difference between spinning our wheels and turning of the wheel.
It's truly walking the razor's edge, as it were, neither suppressing annoyance nor indulging it, but instead watching it with as much mindfulness, equanimity, and compassion as I can muster. From that point, thinking and responding with metta is much more possible.
One of my favorite teachers is Christina Feldman, who said, "The quality of mindfulness is imbued with warmth, compassion, and interest. In the light of this engaged attention we discover it is impossible to hate or fear anything or anyone we truly understand."
Great post, my friend. It seems so simple to me as well, not that i can do what you have described effectively all the time, but whether here online or with my family, coworkers, students, store clerks, folks walking down the streets, it all seems like the same situation, with the same opportunities and challenges. Choose to make the effort to cultivate wholesome qualities of speech, action and mind or be lazy (unmindful) and fall into delusion (greater suffering). Simple situation, incredibly challenging. And we get like a hundred opportunities every day, moment after moment after moment. You have to watch your own mind, observe how you are thinking, feeling, talking, acting and make the necessary course adjustments, again and again.
Some people believe metta can be expressed many different ways, that a teacher screaming or even physically hitting a student (or their child) can be a way metta is expressed. Perhaps this is true, but i'm doubtful. Buddha described wholesome mindstates very specifically. He described the fetters and hindrances, and anger as being a poison.
If we're not coming from a place of calm, joy, compassion, kindness or find ourselves angry and irritable for me i see that as a sign i need to shut my mouth and cultivate a more wholesome state of mind before engaging with others. Others may disagree, and definitely intentions (why we speak/act a way) are crucial but so are actions (how we act/speak)...
This is not easy, though, at all...!