static ExpressionParsingContext create() {
        if (CACHE.get() == null) {
            CACHE.set(ExpressionParsingContext.builder()
                                              .samples(Lists.newArrayList())
                                              .downsampling(DownsamplingType.AVG)
                                              .scopeLabels(Sets.newHashSet())
                                              .aggregationLabels(Sets.newHashSet()).build());
        }
        return CACHE.get();
    }
ExpressionParsingContext contains states in parsing phase of an expression.
oap-server/analyzer/meter-analyzer/src/main/java/org/apache/skywalking/oap/meter/analyzer/dsl/ExpressionParsingContext.java