metricFactory.createMetricSearch doesn't support empty dimension map

[jakeinater]

Feature scope

MetricFactory

Describe your suggested feature

Declaring CustomMetricGroup as following:

      {
        title: 'Test',
        metrics: [
          metricFactory.createMetricSearch(
            'query',
            { dummyDim: undefined as any as string },
            MetricStatistic.SUM,
          ) as MathExpression,
        ],
      },

Which generates a graph with following search query:

SEARCH('{test_namespace,dummyDim}  EligibleEvent', 'Sum', 60)

As far as I can tell, there is no way to omit the DimensionMap. When I emit the metric, I don't associate any dimension, so I need to associate a dummy dimension, which is a little hacky.

Nit: Also unrelated issue, createMetricSearch returns IMetric, which is incompatible with what the MonitoringFacade expects (Metric | MathExpression) resulting in me needing to cast the return into a MathExpression.

Related code snippet: https://github.com/cdklabs/cdk-monitoring-constructs/blob/e7aa66bc3a051a46bc28d674f7e6bca6f4195a6a/lib/common/metric/MetricFactory.ts#L148-L185