Create service for runtime metrics (#367)

* added span messages

Signed-off-by: Daniel Rammer <daniel@union.ai>

* added endpoints to service

Signed-off-by: Daniel Rammer <daniel@union.ai>

* generated mocks

Signed-off-by: Daniel Rammer <daniel@union.ai>

* removed get task execution metrics rpc

Signed-off-by: Daniel Rammer <daniel@union.ai>

* added EXECUTION_IDLE category

Signed-off-by: Daniel Rammer <daniel@union.ai>

* updated PLUGIN_EXECUTION to PLUGIN_RUNTIME

Signed-off-by: Daniel Rammer <daniel@union.ai>

* removed recorded_at on workflow and node level events

Signed-off-by: Daniel Rammer <daniel@union.ai>

* added docs for task event reported_at field

Signed-off-by: Daniel Rammer <daniel@union.ai>

* removed GetNodeExecutionMetrics endpoint - will implement later if necessary

Signed-off-by: Daniel Rammer <daniel@union.ai>

* updated docs

Signed-off-by: Daniel Rammer <daniel@union.ai>

* added reported_at for node execution events

Signed-off-by: Daniel Rammer <daniel@union.ai>

* fixed typo

Signed-off-by: Daniel Rammer <daniel@union.ai>

* fixed typos and removed dead code

Signed-off-by: Daniel Rammer <daniel@union.ai>

* updated categories

Signed-off-by: Daniel Rammer <daniel@union.ai>

* added workflow setup and teardown categories

Signed-off-by: Daniel Rammer <daniel@union.ai>

* simplified span message and moved to flyteidl.core

Signed-off-by: Daniel Rammer <daniel@union.ai>

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Signed-off-by: Daniel Rammer <daniel@union.ai>