Standardize common metrics labels

[lambdanis]

There are labels shared by multiple metrics, but using different names. Using the same name across metrics would make metrics more intuitive and easier to correlate.

Such shared labels should be defined in a shared library (pkg/metrics most likely), so that they can be easily reused.

Any changes made to the existing metrics must be documented in upgrade notes.

• Use opcode for ops.OpCode (rather than msg_op or op)
• Add op label in addition to opcode with a human-readable string
• Use event_type for EventType (rather than event or type)