feat: schema metadata API support (#37)

* feat: support for schema API code generation

Modifies the gRPC / proto code generation helper script to generate both
the existing ingester service code, and the schema service code.

* test: composable test client + write helpers

Refactors the test helper code into two distinct responsibilities that
can be composed together to build up more complex test cases:

	* openNewDatabase: initialise an IOx client with a random database
	* writeDataset: perform a write to a specified db/table

Previously initialising a client also returned the write destination,
which restricted tests to writing to a single (pre-determined) table /
db.

* feat(metadata): schema API client

Implements a GetSchema() metadata call that returns the set of columns
and their data types for a specific table within a namespace.

Redefines the column data types defined in the proto spec, decoupling
callers from the generated wire format (internal) representation.

Currently fetches all tables for a given namespace and filters the
results for the desired table in the client - this should be replaced
with a more efficient table-orientated schema lookup API once it is
available in IOx:

	https://github.com/influxdata/influxdb_iox/issues/4997

Author: domodwyer

generate_influxdb_iox_protobuf.sh

@@ -29,15 +29,29 @@ git clone \
   https://github.com/influxdata/influxdb_iox \
   gen
 
-filenames=$(find gen/generated_types/protos/influxdata/iox/ingester -name '*.proto')
-protoc \
-  --proto_path=gen/generated_types/protos/ \
-  --plugin protoc-gen-go="$(which protoc-gen-go)" \
-  --plugin protoc-gen-go-grpc="$(which protoc-gen-go-grpc)" \
-  --go_out gen \
-  --go-grpc_out gen \
-  ${filenames}
-
-mv gen/github.com/influxdata/iox/ingester/v1/*.go internal/ingester/
+# Generate Go files for the specified IOx service.
+#
+# $1: the IOx proto directory name to generate code for
+#
+# Generates directories by name from
+#  https://github.com/influxdata/influxdb_iox/tree/main/generated_types/protos/influxdata/iox
+function generate_service {
+  filenames=$(find "gen/generated_types/protos/influxdata/iox/${1}" -name '*.proto')
+  protoc \
+    --proto_path=gen/generated_types/protos/ \
+    --plugin protoc-gen-go="$(which protoc-gen-go)" \
+    --plugin protoc-gen-go-grpc="$(which protoc-gen-go-grpc)" \
+    --go_out gen \
+    --go-grpc_out gen \
+    ${filenames}
+
+  mkdir "internal/${1}" 2>/dev/null || true
+  mv gen/github.com/influxdata/iox/"${1}"/v1/*.go "internal/${1}/"
+  
+  echo "generated ${1}"
+}
+
+generate_service "ingester"
+generate_service "schema"
 
 rm -rf gen