Unexpected text output of metric name containing : and broken histogram name output

[gavin-nia]

This was tested using v1.4.2 of prometheus-metrics-exposition-text-formats.

Issue 1:
If a metric name contains a : it is being output in curly brackets. This seems unexpected and is a behavior change from older versions.

Issue 2:
The "le" label is not separated from the metric name in the case of histograms with no other labels. This is a bug.

Test code (still using the pre 1.0 artifacts here)

Counter
    .build()
    .name("mytest:withcolon_total")
    .help("test counter")
    .register();

Histogram
    .build()
    .name("myhistogram:withcolon")
    .help("test histogram")
    .buckets(new double[]{2, 10, 50, 200, 1_000, 5_000, 10_000})
    .register();

These are output as follows:

{mytest:withcolon_total} 0.0
{myhistogram:withcolon_bucketle="2.0"} 0

Looking at the code, PrometheusTextFormatWriter.writeNameAndLabels has:

    // If the name does not pass the legacy validity check, we must put the
    // metric name inside the braces.
    if (!PrometheusNaming.isValidLegacyLabelName(name)) {
      metricInsideBraces = true;
      writer.write('{');
    }

I feel that this should be calling isValidLegacyMetricName rather than isValidLegacyLabelName. The former does allow : in the name.

For the issue with the histogram, TextFormatUtil.writeLabels should probably be checking metricInsideBraces if an additionalName is present.