// JSON does not support these special values
			if math.IsNaN(fv) || math.IsInf(fv, 0) {

[[processors.starlark]]
  source = '''
load("logging.star", "log")

def apply(metric):
    for k, v in metric.fields.items():
        if type(v) == "float" and str(v) == "NaN":
            metric.fields.pop(k)
            log.warn("Dropped NaN value: metric {} field {}".format(metric.name, k))

    return metric
'''

➜ go test -v -run TestConnectAndWriteMetricWithNaNValue
=== RUN   TestConnectAndWriteMetricWithNaNValue
2021/04/29 20:11:09 I! Elasticsearch version: 7.1.1
2021/04/29 20:11:09 D! Elasticsearch template telegraf created or updated
    elasticsearch_test.go:67: 
                Error Trace:    elasticsearch_test.go:67
                Error:          Received unexpected error:
                                Error sending bulk request to Elasticsearch: json: unsupported value: NaN
                Test:           TestConnectAndWriteMetricWithNaNValue
--- FAIL: TestConnectAndWriteMetricWithNaNValue (0.13s)
FAIL
exit status 1
FAIL    github.com/influxdata/telegraf/plugins/outputs/elasticsearch    0.133s