Is it not enough to practice scales alone?
If you just want to learn the solos of other players, you don't need any theory, and you don't need to learn any chords. I'm just answering because everyone else it saying that you do. It's not true, a lot of people want to learn guitar because they like the lead guitar parts they hear their favorite artists play. You can play the lead parts of a song without even knowing what the actual notes are, and what key the song is in...
When I started playing, I wasn't interested at all in chords, because I thought that all the interesting bits where in the melody, and that people would rather hear the solo played alone, instead of just the rhythm. Imo, you'll want to learn chords pretty soon even if your main objective is to master lead guitar. If you're not convinced that you should learn chords, than don't. You can play lead without them, but I'm convinced you'll learn them sooner or later. Many lead players rely heavily on arpeggios, and arpeggios are just chords played one note at a time.
How well should I know chords?
Unless you want to be able to improvise or compose your own songs/solos, you do not need to know any chords. You just need to pick up the tablature and practice. The only advantage of knowing chords there, is it helps you memorize arpeggios better. For example instead of remembering the fret positions of the notes A C E G, you just remember that it's an A minor 7 arpeggio.
If your goal is to compose or improvise, learning all the basic concepts of music theory (chords, scales, keys, rhythm...) is very helpful. In fact it's very hard to do anything without having at least a basic understanding of all of these concepts.
What should I practice everyday?
Again, it depends on your goals:
If your goal is just to be able to play you favorite Metallica solos, just practice scales and the actual solos you want to learn. If you're curious about how they work, why they sound nice, or how to make something similar, you probably have to learn all the basic theory (not just the scale) and do some ear training exercises.
Say, if I like to be able to play lead of some of my favorite Metallica tracks? will I need to know the scales and chords that are used?
No, I know a lot of players who have no idea in what key they're playing, or what the chords are ; and yet they play the parts they practice extremely well, and I really enjoy listening to them. However, scales do help a lot because most (if not all) of the notes you'll be playing are going to be part of the scale. So scales help you memorize. Chords won't help you as much in this particular situation, but they can still help you in some areas, like memorizing arpeggios.
My advice: for now, just learn and practice tablature. Learn some of your favorite lead parts. Do just what you feel like doing. Don't worry about chords and theory. In the end this just a hobby, and it's meant to be fun. Why do you want the answers to questions you're not even asking? You're obviously not interested in chords right now. And you also don't need them to learn to play a solo. In the long run, if you're still as enthusiastic and motivated as you are right now, you'll start asking those questions automatically.