Add support for list values in tags in Span and SpanCustomizer

[Jasz]

Currently, Span and SpanCustomizer support tags with String, boolean, long and double values. OpenTelemetry additionally supports lists of those four types. This feature request proposes adding support for lists of these types to Micrometer.

Unfortunately, it seems that Brave doesn't support lists. One solution would be to concatenate the lists to a String.