Update to OpenTelemetry-cpp release 1.12.0

[duncanpo]

Currently at 1.11.0.
1.12.0 has a few useful updates:
2298 is a bug fix necessary to implement asynchronous counters
2304 may be necessary to adopt std features (#19)
2342 drops C++11 support and moves to at least C++14
2284 lifts a restriction to metric name length

[duncanpo]

1.12.0 contains 2 major issues:

1. Namespace issues in header files opentelemetry/sdk/metrics/data/exemplar_data.h and opentelemetry/sdk/metrics/exemplar/reservoir_cell.h result in build failure
2. UpDownCounter is broken, cannot count down.

Both problems have been fixed subsequently, in commits 27a58cd and d9ad76f respectively. Given these, we should skip this release and wait for the next.