Allow multiple definitions as fallback in `metadata-service-mapping.yaml` file

[sergicastro]

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Requirement or improvement

Regarding the metadata-service-mapping.yaml file used to configure which fields/labels are used to extract required data, would be nice to provide a way to configure multiple values treated as a fallback mechanism to extract the required data.

For example, let's say I want to extract the service name from the label service.istio.io/canonical-name but I want to fall back into app.kubernetes.io/name or app labels if the first one is not defined.
Allowing a comma-separated value in the config file would be enough to what needs to be processed to find the service name in this case.
The config file would look like this:

serviceName: ${LABELS."service.istio.io/canonical-name"},${LABELS."app.kubernetes.io/name"},${LABELS."app"}
serviceInstanceName: ${NAME}

This is just a suggestion, any other implementation covering the requirement is welcome.

[kezhenxu94]

The config file would look like this:

serviceName: ${LABELS."service.istio.io/canonical-name"},${LABELS."app.kubernetes.io/name"},${LABELS."app"}
serviceInstanceName: ${NAME}

This is just a suggestion, any other implementation covering the requirement is welcome.

Instead of ${a},${b},${c}, I used ${a,b,c} in #6933 , so something like prefix::${a,b,c}::suffix will work if a is empty but b is not (say test), it evaluates as prefix::test::suffix, using ${a},${b},${c} will need to duplicate the prefixes and suffixes like prefix::${a}::suffix,prefix::${b}::suffix,prefix::${c}::suffix, and the empty check is even complicated since a == null evaluates prefix::${a}::suffix as prefix::null::suffix