UTF-8 support in metric and label names

[fedetorres93]

Adds UTF-8 support for metric and label names.

These changes are based on the work done on the Prometheus common libraries here and here

• The prometheus-metrics-exposition-formats module will use the new quoting syntax {"foo"} iff the metric does not conform to the legacy name format (foo{})
• The prometheus-metrics-model module has a new flag (NameValidationScheme) that determines if validation is done using the legacy or the UTF-8 scheme. This flag can be set via a property in the properties file.
• Scrapers can announce via content negotiation that they support UTF-8 names by adding escaping=allow-utf-8 in the Accept header. In cases where UTF-8 is not available, metric providers can be configured to escape names in a few different ways: values (U__ UTF value escaping for perfect round-tripping), underscores (all invalid chars become _), dots (dots become _dot_, _ becomes __, all other values become ___). Escaping can either be a global default (PrometheusNaming.nameEscapingScheme) or can also be specified in Accept header with the escaping= term, which can be allow-utf-8 (for UTF-8-compatible), underscores, dots, or values.
  This should still be a noop for existing configurations because scrapers will not be passing the escaping key in the Accept header. Existing functionality is maintained.
• The prometheus-metrics-exporter-pushgateway module will escape UTF-8 grouping keys in the URL path used when pushing metrics (see Add UTF-8 support in metric and label names pushgateway#689)

Work towards prometheus/prometheus#13095

[fedetorres93]

Given the ongoing discussion about unit suffixes for OM 2.0 (prometheus/OpenMetrics#286), I think we can take this UTF-8 work as a basis and then add the necessary changes to comply with the final consensus on suffixes.