[pkg/telemetryquerylanguage] Add `drop` action support for metrics

[dmitryax]

[github-actions]

Pinging code owners: @TylerHelmuth @kentquirk @bogdandrutu. See Adding Labels via Comments if you do not have permissions to add labels yourself.

[TylerHelmuth]

Metrics context already has access to the datapoint's metric and scope metrics slice so it is ready to be used with a general drop function

[dmitryax]

@TylerHelmuth should this be closed given that drop action is not supported yet?

[TylerHelmuth]

I slightly modified these subtasks to reflect the context's abilities to handle a function like drop. The new drop function will be generic to any context and can get registered to the transform processor's common function registry once each context has access to the telemetry slice.