You are right,

Some first level spells are not (that) useful for first level characters

The combat rules for measuring the duration of effects (including spells) state:

When the rules refer to a "full round", they usually mean a span of time from a particular initiative count in one round to the same initiative count in the next round. Effects that last a certain number of rounds end just before the same initiative count that they began on.

^{(emphasis mine)}

Unfortunately for our first level casters, this means that when they cast a spell which lasts only 1 round, it will fizz away immediately before they get a chance to use it.

So, why would something like this even be a Level 1 option?

First of all, there are several exceptions which makes such spells useful even for a first level character:

• Touch spells allow you to "hold the charge" until you successfully hit with them, and give you a "free" touch attack attempt as part of the action of casting the spell.
  So you can attack on the same round you cast them, as well as keep trying to hit for as many following rounds as you need.
• The spell is still active during other combatants' actions on that round - both enemies and allies - this may come up when:
  • Your spell can counter an enemy's action - for example, casting Unbreakable Heart on yourself will protect you from a Fear spell your enemy will cast during his action.
  • You cast the spell to buff an ally (such as Sun Metal cast on your barbarian friend's axe or Unbreakable Heart cast on a confused ally, allowing him to take his turn normally)
  • An enemy exposes himself to your attack of opportunity (your Sun Metal will be active and enhance your damage during his turn)

But, most importantly, some first level spells are mainly useful for casters of higher levels - but it is still better that they are first level spells, because:

• You can access them earlier - for example, Sun Metal may be cast by a 5th level ranger (caster level 2), if it was a second level spell, that ranger will have to be at least 7th level to cast it.
• You can cast them more times per day - sticking with Sun Metal as an example - a 7th level ranger can cast it 2 times per day, instead of just once if it was a second level spell.
• They don't "compete" for spell slots with the more powerful second level spells - at the end of the day, you probably wouldn't want to trade a casting of Brow Gasher or Wilderness Soldiers, just to gain the benefits of Sun Metal - it is not powerful enough to justify being a second level spell, and if it was, it probably would have never been selected by any ranger.

Hope this helps you making sense of it all...

p.s. - I'm not even trying to discuss whether casting Sun Metal is ever the optimal choice for a ranger of any level - you didn't seem to ask about that, and I normally don't care much about such issues myself, so I may not be the best user to tackle that...

The rules would have it do so, yes

The sorcerer’s elemental bloodline literally does nothing but swap the damage type of the spell; all other effects remain the same. In effect, an acid burning hands still launches a jet of flame and still sets things on fire – it’s just that the fire and flame now deal acid damage instead of fire damage. This can, of course, be easily justified as, “hey, it’s magic.”

Of course, seeing as acid’s effect is frequently described as a burning sort of feeling, this isn’t much of a stretch – the action to “put out the fire” could be “wiping the acid off” and “dousing” it could be “diluting” it. In most cases, “being on fire” is entirely equivalent to just “taking damage-over-time that you can use an action to end.” But then if you consider a spell that magically puts out fires – which would work – it gets a little tougher to explain.

It doesn’t have to be this way, of course. The DM certainly has purview to change something like this, and there is certainly room for adding variety this way. The problem is that the game does not define obvious substitutions to use; fire damage sets things on fire, but it’s less clear what the rest should do if you want them to all be distinct. Even setting up distinct damage-over-time effects is awkward – some spells douse fires, but there won’t be any spells that end these new damage-over-time effects that you make up.

Which is, of course, why the rules don’t do this: having a unique status effect tied to each energy damage type requires having the rest of the game react to that fact. Pathfinder doesn’t, and adding it now would be very difficult.¹ It’s much easier to say “it’s acid-damage-dealing fire, deal with it; magic, yo,” than to write out specific versions for each element, and then make sure they’re all reasonably competitive with one another, and that the rest of the product line takes into account, and so on.

1. ^{Remember, even when the elemental bloodline was written, much of “Pathfinder” was already “written” – the core rules did not change that much from 3.5, and most of 3.5 was just used as-is. A change like this would have required rewriting substantial portions of 3.5 that they chose to leave alone, and thus even at the “beginning” would have been very difficult.}