Add table of contents in api-metrics.md (#402)
Add table of contents in api-metrics.md (#402)

Signed-off-by: Daniel González Lopes <5c83c9ad8b47a302ebc96fb9b7d6f7f04324f984@gmail.com>

Co-authored-by: Bogdan Drutu <315ccf64d5700fce428b26783eedef2f04b45764@google.com>

open-telemetry/opentelemetry-specification,open-telemetry/opentelemetry-specification