kmsstats: Fix processing of latency metada
kmsstats: Fix processing of latency metada

when multiple metas with the same API are attached to the same buffer
only the first one was taken into account. This patch iterates over
all metas, so if a muxer copies several latency metas (in case a new
package with audio and video is composed) all latency metas will be
taken into account

Change-Id: I0173ee6b116b8895462b2146502194b1fb69a754

ESTOS/kms-core,Kurento/kms-core,ESTOS/kms-core,Kurento/kms-core,ESTOS/kms-core,ESTOS/kms-core,Kurento/kms-core,Kurento/kms-core