Resolve open-telemetry#4201

The specification requires the duplicate instrument conflicts to be
identified based on the instrument identifying fields:

- name
- instrument kind
- unit
- description
- language-level features such as the number type (int64 and float64)

Currently, the conflict detection and aggregation caching are done based
on the stream IDs which include an aggregation name, monotonicity, and
temporality instead of the instrument kind.

This changes the conflict detection and aggregation caching to use the
OpenTelemetry specified fields. This is effectively a no-op given there
is a 1-to-1 mapping of aggregation-name/monotonicity/temporality to
instrument kind (they are all resolved based on the instrument kind).

Additionally, this adds a stringer representation of the
`InstrumentKind`. This is needed for the logging of duplicate instrument
conflicts.