[GCP] Add CrossSeriesReducer support in ListTimeSeries request

[Linu-Elias]

Add reducer field to the GCP configuration to aggregate data points from multiple time series into a single time series.

A Reducer operation in ListTimeSeries request should aggregate data points from multiple time series into a single time series, where the value of each data point in the resulting series is a function of all the already aligned values in the input time series.
Default value is REDUCE_NONE but other values supported are REDUCE_MEAN, REDUCE_MIN, REDUCE_MAX etc.

Please check https://cloud.google.com/monitoring/api/ref_v3/rpc/google.monitoring.v3#reducer[Reducers] for the full list.

[botelastic]

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