docs(instrumenting): Update exposition_formats.md to include quoting syntax

docs/instrumenting/exposition_formats.md

@@ -54,24 +54,41 @@ line may exist for any given metric name.
 
 If the token is `TYPE`, exactly two more tokens are expected. The first is the
 metric name, and the second is either `counter`, `gauge`, `histogram`,
-`summary`, or `untyped`, defining the type for the metric of that name. Only
-one `TYPE` line may exist for a given metric name. The `TYPE` line for a
-metric name must appear before the first sample is reported for that metric
-name. If there is no `TYPE` line for a metric name, the type is set to
-`untyped`.
+`summary`, or `untyped`, defining the type for the metric of that name. Only one
+`TYPE` line may exist for a given metric name. The `TYPE` line for a metric name
+must appear before the first sample is reported for that metric name. If there
+is no `TYPE` line for a metric name, the type is set to `untyped`. Metric names
+not corresponding to the legacy Prometheus metric name character set must be
+quoted and escaped.
 
 The remaining lines describe samples (one per line) using the following syntax
 ([EBNF](https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form)):
 
 ```
-metric_name [
-  "{" label_name "=" `"` label_value `"` { "," label_name "=" `"` label_value `"` } [ "," ] "}"
-] value [ timestamp ]
+metric_name_or_labels value [ timestamp ]
+
+metric_name_or_labels = metric_name [ "{" labels "}" ] | "{" quoted_metric_name [ "," labels ] "}"
+
+metric_name = identifier
+
+quoted_metric_name = `"` escaped_string `"`
+
+labels = [ label_pairs ]
+
+label_pairs = label_pair { "," label_pair } [ "," ]
+
+label_pair = label_name "=" `"` escaped_string `"`
+
+label_name = identifier | `"` escaped_string `"`
 ```
 
 In the sample syntax:
 
-*  `metric_name` and `label_name` carry the usual Prometheus expression language restrictions.
+* `identifier` carries the usual Prometheus expression language restrictions.
+* `escaped_string` consists of any UTF-8 characters, but backslash, double-quote, and line feed must be escaped.
+* When `metric_name` is quoted with double quotes, it appears inside the braces instead of outside.
+* `label_name` may be optionally enclosed in double quotes.
+* Metric and label names not corresponding to the usual Prometheus expression language restrictions must use the quoted syntaxes.
 * `label_value` can be any sequence of UTF-8 characters, but the backslash (`\`), double-quote (`"`), and line feed (`\n`) characters have to be escaped as `\\`, `\"`, and `\n`, respectively.
 * `value` is a float represented as required by Go's [`ParseFloat()`](https://golang.org/pkg/strconv/#ParseFloat) function. In addition to standard numerical values, `NaN`, `+Inf`, and `-Inf` are valid values representing not a number, positive infinity, and negative infinity, respectively.
 * The `timestamp` is an `int64` (milliseconds since epoch, i.e. 1970-01-01 00:00:00 UTC, excluding leap seconds), represented as required by Go's [`ParseInt()`](https://golang.org/pkg/strconv/#ParseInt) function.
@@ -113,6 +130,9 @@ http_requests_total{method="post",code="400"}    3 1395066363000
 # Escaping in label values:
 msdos_file_access_time_seconds{path="C:\\DIR\\FILE.TXT",error="Cannot find file:\n\"FILE.TXT\""} 1.458255915e9
 
+# UTF-8 metric and label names:
+{"my.dotted.metric", "error.message"="Not Found"}
+
 # Minimalistic line:
 metric_without_timestamp_and_labels 12.47