Can matastases spread in different parts of the body of an animal suffering from cancer grow in a limited way, such as not to cause much more harm that a benignant tumor in those parts of the body would, or are metastases guaranteed to eventually cause death, if left uncured?
What you're asking is essentially to prove a negative ("Are there no ways an unchecked cancer can be non-lethal?"), which is unfortunately more of an exercise in imagination than anything else. The best way I can answer is by highlighting what about cancer actually kills, from which you can personally evaluate if a particular type of cancer fits and would be lethal.
You need to remember that it is not the metastization per se that kills something, it is the growth interfering with critical systems that causes death. Many systems in a body will prove lethal if sufficiently compromised. The brain is an obviously vulnerable point (a sample of one of its mechanisms can be found here), but even something like the lymphatic system, which is notably able to maintain its duties with some components compromised or removed, are ultimately necessary (a compromised lymphatic system will, among other things, leave one vulnerable to otherwise harmless infections). The primary checks to take away from this are "Can a body survive with this system compromised, or to what degree?".
Even in the case of a hypothetical cancer that is not particularly metastatic, but simply growing in a particular area (we'll even be generous and say the particular area is not vital to survival), tumors take resources to maintain, both chemically and mechanically (pulmonary issues are known to occur with this sort of unguided growth). Many of these effects can be highlighted in overgrowth diseases such as in Proteus syndrome (Note - My apologies, this is behind an academic paywall). Considerations for this part are "How many resources can a body afford to lose?" and "Can the heart support this extra mass?".
As an additional factor, tumors that don't allow infiltration by cells from the immune system can be prone to infections, both internally and externally. This is circumstantial, though, and not a core component of a cancer's lethality (mostly).
The mere presence of a tumor is not guarenteed to be lethal, but I would say, ultimately, any continuously-growing tumor will lead to death, simply by subverting the proper mechanisms of the body to a sufficient level.