[MONIT-4106] fluentbit/metrics: add support for user provided lua scripts

While doing trying to mimic the OpenShift logging setup with the current
Helm chart we noticed that they were using the lua scripts input from
the fluent bit official Helm chart. In our case this was not possible
and had to be done by adding secrets one by one.

We agreed to mimic this behaviour and had our own helm chart have the
option to add lua scripts to the fluent bit via helm chart values.

This commit modifies the configmap used to create the lua script that
we required for the timestampt in such a way that now it creates this
first lua script but also creates any other provided by the users via
the values of the helm chart.

Reported-at: https://its.cern.ch/jira/browse/MONIT-4106


Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@gmail.com>
Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@cern.ch>