Add spanmetricsprocessor logic (#2012)

**Description:** Adds spanmetricsprocessor logic as a follow-up to #1917.

**Link to tracking Issue:** #403

**Testing:**
- Unit and integration tested locally (by adding to `cmd/otelcontribcol/components.go`).
- Performance tested (added metrics key cache to improve performance) and added benchmark function.

**Documentation:** Add relevant documentation to exported structs and functions.

processor/spanmetricsprocessor/README.md

@@ -1,5 +1,8 @@
 # Span Metrics Processor
 
+**Note:** Currently experimental and subject to breaking changes (e.g. change from processor to exporter/translator component).
+See: https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/403.
+
 Aggregates Request, Error and Duration (R.E.D) metrics from span data.
 
 **Request** counts are computed as the number of spans seen per unique set of dimensions, including Errors.

processor/spanmetricsprocessor/config.go

@@ -20,11 +20,13 @@ import (
 	"go.opentelemetry.io/collector/config/configmodels"
 )
 
+// Dimension defines the dimension name and optional default value if the Dimension is missing from a span attribute.
 type Dimension struct {
 	Name    string  `mapstructure:"name"`
 	Default *string `mapstructure:"default"`
 }
 
+// Config defines the configuration options for spanmetricsprocessor.
 type Config struct {
 	configmodels.ProcessorSettings `mapstructure:",squash"`