monitoring: filter_drop: typo and description

[fdroletlmi]

The metric is named fluentbit_filter_drop_records_total, as per https://github.com/fluent/fluent-bit/blob/9652b0dc6fd9e59ef87c3bdc59c89da9abed995b/include/fluent-bit/flb_filter.h#L108

[github-actions[bot]]

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