Change recently added metric key name. (very simple change)

[upgle]

This PR is related to #2968(merged 2days ago) and thank you for your commit @mhenke1

I think it is more common to use dotted notation format(.) for recording metric.

Most time series db like Graphite, OpenTSDB and InfluxDB normally use a dot-delimited notation. (e.g. sys.cpu.user, invoker_docker.rm_start)

Even in the openwhisk code, metric keys for recording docker cmd use dot delimiter.

def INVOKER_RUNC_CMD(cmd: String) = LogMarkerToken(invoker, s"runc.$cmd", start)

If you are using kamon-statsd with graphite, a query is a better way.
For example, a query like invoker_containerStart.prewarmed.* will fill the variable with all possible values that exist in the wildcard position

Before

invoker_container_start_prewarmed_guest_hello_count

if action name is foo_bar and namespace is baz_qux then this marker will look ambiguous. invoker_container_start_prewarmed_baz_qux_foo_bar_count

After

invoker_containerStart.prewarmed.guest.hello_count

but in this case, dotted format is not ambiguous and can be seperated by dot(.) delimiter in Grafana, invoker_containerStart.prewarmed.baz_qux.foo_bar_count

with grafana, It is possible to query using wildcard(*) easily invoker_containerStart.prewarmed.*

And I also changed naming convention,
because most of the metric key names already used in the code is using camelcase like activationRun, invokerOffline and blockingActivation,

[markusthoemmes]

@mhenke1 would you please review?

[upgle]

I will close this PR because these metrics removed from #3284 PR.