You can use a qualified association:
┌──────────┐ 1 ┌───────┐
│ MyServer │Key│───────────│ Value │
└──────────┘ └───────┘
See: http://etutorials.org/Programming/UML/Chapter+6.+Class+Diagrams+Advanced+Concepts/Qualified+Associations/ (cause it is hard to draw using ASCII)
Note also that a qualified association changes the multiplicity:
┌──────────┐ 0..* ┌───────┐
│ MyServer │───────────────│ Value │
└──────────┘ └───────┘
┌──────────┐ 1 ┌───────┐
│ MyServer │Key│───────────│ Value │
└──────────┘ └───────┘
The top illustrates an association from a server to 0-n values. By contrast, the qualified association says that any given key will be associated with only one value, and that you can’t have a key an absent value.