Breaking down some more components, adding tests.

* Some bug fixes found during testing/test repair.
* Trying to tease apart the various responsibilities of `metricNamer` into smaller chunks and adding tests for each individual chunk.
* Updating the `provider_test` and `series_registry_test` to fix failures.

pkg/custom-provider/query_builder.go

@@ -0,0 +1,96 @@
+package provider
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+	"text/template"
+
+	prom "github.com/directxman12/k8s-prometheus-adapter/pkg/client"
+)
+
+type QueryBuilder interface {
+	BuildSelector(seriesName string, groupBy string, groupBySlice []string, queryParts []queryPart) (prom.Selector, error)
+}
+
+type queryBuilder struct {
+	metricsQueryTemplate *template.Template
+}
+
+func NewQueryBuilder(metricsQuery string) (QueryBuilder, error) {
+	metricsQueryTemplate, err := template.New("metrics-query").Delims("<<", ">>").Parse(metricsQuery)
+	if err != nil {
+		return nil, fmt.Errorf("unable to parse metrics query template %q: %v", metricsQuery, err)
+	}
+
+	return &queryBuilder{
+		metricsQueryTemplate: metricsQueryTemplate,
+	}, nil
+}
+
+func (n *queryBuilder) BuildSelector(seriesName string, groupBy string, groupBySlice []string, queryParts []queryPart) (prom.Selector, error) {
+	//Convert our query parts into the types we need for our template.
+	exprs, valuesByName := n.processQueryParts(queryParts)
+
+	args := queryTemplateArgs{
+		Series:            seriesName,
+		LabelMatchers:     strings.Join(exprs, ","),
+		LabelValuesByName: valuesByName,
+		GroupBy:           groupBy,
+		GroupBySlice:      groupBySlice,
+	}
+
+	selector, err := n.createSelectorFromTemplateArgs(args)
+	if err != nil {
+		return "", err
+	}
+
+	return selector, nil
+}
+
+func (n *queryBuilder) createSelectorFromTemplateArgs(args queryTemplateArgs) (prom.Selector, error) {
+	//Turn our template arguments into a Selector.
+	queryBuff := new(bytes.Buffer)
+	if err := n.metricsQueryTemplate.Execute(queryBuff, args); err != nil {
+		return "", err
+	}
+
+	if queryBuff.Len() == 0 {
+		return "", fmt.Errorf("empty query produced by metrics query template")
+	}
+
+	return prom.Selector(queryBuff.String()), nil
+}
+
+func (n *queryBuilder) processQueryParts(queryParts []queryPart) ([]string, map[string][]string) {
+	//Contains the expressions that we want to include as part of the query to Prometheus.
+	//e.g. "namespace=my-namespace"
+	//e.g. "some_label=some-value"
+	var exprs []string
+
+	//Contains the list of label values we're targeting, by namespace.
+	//e.g. "some_label" => ["value-one", "value-two"]
+	valuesByName := map[string][]string{}
+
+	//Convert our query parts into template arguments.
+	for _, qPart := range queryParts {
+		//Be resilient against bad inputs.
+		//We obviously can't generate label filters for these cases.
+		if qPart.labelName == "" || len(qPart.values) == 0 {
+			continue
+		}
+		targetValue := qPart.values[0]
+		matcher := prom.LabelEq
+
+		if len(qPart.values) > 1 {
+			targetValue = strings.Join(qPart.values, "|")
+			matcher = prom.LabelMatches
+		}
+
+		expression := matcher(qPart.labelName, targetValue)
+		exprs = append(exprs, expression)
+		valuesByName[qPart.labelName] = qPart.values
+	}
+
+	return exprs, valuesByName
+}